Telnet

FYI the commands below is for Ubuntu OS

-Check status:

sudo /etc/init.d/xinetd status

or

sudo service telnet status

Restart telnet server:

sudo /etc/init.d/inetd restart

or

sudo /etc/init.d/openbsd-inetd restart

If neither xinetd, inetd nor openbsd-inetd was found (if all three doesnt exist) then telnet is not installed

Look for telnet solution directory with:

sudo find / -name telnet

or

apt-cache policy telnet -In ubuntu 16.04 will see that it is installed but if all the above doesnt work then it means it wasnt enabled.

or 

telnet -V 

or 

telnet –V

If you dont have inetd deamon wont be able to use the telnet service in your system. When you install telnetd deamon it also installs inetd deamon.

The telnet service is pre-installed in ubuntu but because default ubuntu has no listening port the telnet service is not useful yet. 

That also means that one does not need to disable telnet unless the one have installed a telnet daemon

Reference:

https://wiki.ubuntu.com/SecurityTeam/Policies#No_Open_Ports

https://www.ibm.com/support/knowledgecenter/zosbasics/com.ibm.zos.znetwork/znetwork_112.htm

http://www.ibm.com/support/knowledgecenter/ssw_aix_72/com.ibm.aix.cmds5/telnetd.htm

http://www.cyberciti.biz/faq/how-do-i-turn-on-telnet-service-on-for-a-linuxfreebsd-system/

http://askubuntu.com/questions/389298/how-do-i-disable-telnet-on-ubuntu-server

How Email Works

How Does Email Work? A Simple (Illustrated) Explanation

How Does Email Work?

How Email Works

Exactly How Emails Works – Steps and Explanation

How Email Really Works

How Does An Email Server Work? [Technology Explained]

How E-mail Works

HOW E-MAIL WORKS

How Internet Email Works

How Email Works

How Email Works

How Email Works

How email works (MTA, MDA, MUA)

How email works

How does email work?

How E-mail Works

SMTP Inside Out

How does email work?

How does the sending of email work?

Email

How Does the Email System Work?

How Email Works

How Does Email Work?

How Does Email Work In Plain English So That A 5th Grader Can Understand It

How email works

how does email work?

How does Email Work?

How incoming email works

How email works

MTA, POP, IMAP?

Message transfer agent

What are IMAP and POP?

Email: What’s the Difference Between POP3, IMAP, and Exchange?

Post Office Protocol

Read Gmail messages on other email clients using POP

What Is POP & IMAP and Which One Should You Use for Your Email?

What is the difference between POP3 and IMAP?

Email server settings: POP and IMAP

How to set up a POP/IMAP email account in Microsoft Outlook 2016

Set up your email address with POP

Difference Between POP3 and IMAP E-mail Accounts

Internet Message Access Protocol

IMAP (Internet Message Access Protocol)

What is a IMAP?

Use IMAP to check Gmail on other email clients

How to set up a POP/IMAP email account in Microsoft Outlook 2013

What is the difference between POP and IMAP?

IMAP vs POP3 vs SMTP

How to email in the 21st century

What is an MX Record and how to look up a server/domain/dns server MX Record or MX record ip

check dns mx record for a domain and how to look up mx record for a domain on command line?

Why can’t MX records point to an IP address?MX record

What is an MX record?

Email Fundamentals: What is an MX Record, and How Do They Work?

What is an MX record, and how is it created?

DNS Management – How to Modify MX Records

How do I add or edit mail or MX records?

MX Record Tutorial

What is an MX record?

Changing your MX records

A-record and MX-record: how does it work?

Adding or Updating MX Records in Your Wix Account

Modifying your MX records

Mail Exchange Record, what to put for your MX record

What Is An MX Record? What Is The Correct Syntax For MX Records? What Is Priority?

MX record

Edit/Change the Domain’s Mail Servers (MX Record)

How to Use Nslookup to Verify MX record configuration

MX Lookup Online

MX LOOKUP – Find the mail exchanger for an hostname

Check your DNS records with dig

How to Lookup the MX record(s) for a Domain

Check Your DNS MX Records

Query a Domain Name Server and display the results online

Get MX record for domain name

How do I get the mx records for a domain from *nix command line?

DNS Lookup

DNS Propagation Checker

Add an MX record

DNS Records for custom domains (Verify & MX Record)

Check DNS Resolution

How do you use NSLOOKUP with MX records?

How to check domain’s MX ( mail exchange ) records using dig command on Linux

Check DNS records on Windows with nslookup

How to troubleshoot DNS with dig and nslookup

How to Check MX record using Windows Command Prompt

How to Check MX Record Using nslookup Command on Windows

3 Easy Ways To Check DNS (Domain Name Server) Records In Linux Terminal

How to use nslookup to query MX records (was:Dns)

Command line tool to view MX record

How to do a MX lookup on windows

Dos Prompt Home Page Using the Command Prompt MX Record and Telnet Example

How To Use NSLOOKUP to View Your DNS Records

getmxrr

LOOKING UP MX RECORDS

How To Check DNS Records Using Basic Nslookup Command Examples

Understanding email routing (i.e. MX-records)

How to Find the SMTP Email Server for an Email Address

How to: Check DNS Records Using NSLOOKUP In Windows XP

Nslookup – Common Usage Examples

Windows NSLookup MX Record Lookup 2

10 DNS questions solved with nslookup

MX record lookup command line

Checking FQDN, Reverse-DNS/PTR, MX record

Using Nslookup in Windows to Check DNS Records

How to Use Nslookup – Beginners Guide

How to Use Nslookup to Verify MX record configuration

What is an MX Record?

MX (Mail eXchange) FAQ

MX record (Mail Exchange record)

What is an MX-record? How do I create / change it?

MX Record

MX Record

Mail Exchange Record, what to put for your MX record

MX record

What are MX Records?

MX Record in DNS Explained with Example Configurations

What is an MX-record (Mail eXchanger)?

What is MX Record? Why is it Important?

What are Mail Exchanger (MX) Records and how do I edit/add them?

What is an MX record?

Managing Advanced DNS Records

What is an MX record and how do I add one?

What Is An MX Record?

What is an “MX” record?

What Is The MX Record – Mail Exchange

What are MX records, and why do I need this to run an e-mail server?

Understanding MX records and A records

What is an MX record?

MX records

How to manage Mail Exchanger (MX) records in cPanel

MX-record setup

Basics of an MX Record Cutover

What is an MX record?

What is a CNAME, A-Record or MX-Record and how can I register it?

What is an MX Record?

What should my MX records be?

What is an MX Record

What is MX record priority(preference)

What is an MX record?

How to change MX records in cPanel

How can I set up MX records required for mail service?

What is an MX record?

What is an MX Record?

Portal: Manage DNS Mail eXchange (MX) Records

What is a mx record?

AN INTRODUCTION OF MX RECORDS PRIORITY

Custom DNS: MX Records

WHAT IS AN MX RECORD?

Email & MX Records

What Is An MX Record? (And How To Add It)

MX RECORDS

Do you need an MX record for a hostname?

Whats an MX record and how does it work?

What is an MX record in DNS?

MX records explained

MX Records

Undestanding MX records, Mail Relay/Authentication and Open Relay

MX Record

What is an MX record?

What is an MX Record?

What is a MX Record?

How to configure my MX record for 2-way Email

What is an MX record?

What is an “MX” record

MX Records Explained- Beginners Guide

MX record priority

What is MX Record, how many XR Record are there?

check dns mx record for a domain and how to look up mx record for a domain Using Powershell?

Distribution Group and Powershell

Manage Distribution Groups by using PowerShell | Office 365

Manage Distribution Group using PowerShell in Office 365 | Adding members to existing Distribution Group | Part 3#5

Question:How To Add Users To A Distribution Group From A .CSV File In Exchange Server 2010

Add Users to a Distribution Group from a CSV File (Exchange 2010)

Add multiple members to a distribution group in O365

Exchange 2010/2013 Bulk Add Members to Distribution Groups From CSV

How to add multiple users (email address) to an Exchange 2010 distribution group via Powershell V1.0

BulkAddMemberstoDLs

Powershell 2: Add multiple members to distribution group with one call?

Add bulk users to Distribution Group using Powershell Cmdlet

Add users to a distribution group from a .csv file in exchange 2007\2010 Powershell

Exchange Server 2010 Windows PowerShell: Working with Distribution Groups

Tech Tip | Creating Office 365 Distribution Groups by CSV and Adding Members by CSV

Bulk add users to Distribution Group

is ADD-distributiongroupmember a cmdlet in powershell to add members into a distribution list

How can I add members to a new distribution group from a CSV file?

Bulk add members to a distribution list

Add Mailbox Users to Distribution Lists with PowerShell

Add multiple users to multiple groups from one import csv (follow up query)

Import distribution groups to Office 365: Exchange Online with PowerShell

Two Ways to Add Multiple Users or Contacts to a Distribution Group

point mailbox to another AD account

How to attach existing user mailbox to a new user in different domain

Understanding the Mailbox Move Request in Exchange 2010

Moving Mailboxes the Exchange 2010 Way

How to move mailbox from one Exchange 2010 to another (different domain)?

Moving Mailboxes in Exchange 2010 (Part 1)

How to Move Mailboxes in Exchange Server 2010

Easiest method to move Exchange mailboxes to new server?

Migrating Exchange 2010 mailboxes between domains how to….

How to change owner of a mailbox

Re-assign an exchange 2010 mailbox to another user

assign existing mailbox to a new user account

exchange 2010 reasign mailbox to another user

Re-assign an exchange 2010 mailbox to another user

Re-assign an exchange 2010 mailbox to another user

Exchange 2010 – Link mailbox with another AD account

How can I change which AD account an Exchange mailbox is connected to without deleting the account?

how to I assign an existing mailbox to a new user in Exchange 2010

Moving a Exchange 2007/2010 mailbox to another user

Move Exchange Mailbox Data “User A” to “User B” – User “A” got fired

Create new AD user accounts for all users, keep old mailbox/email address

Changing account of linked mailbox

Re-assign existing mailbox to new user

Existing mailbox assign to another user

moving mailboxes from one AD account to another – GUI

link old mailbox to New AD user account

HOW TO: Reconnect a mailbox to another user

Understanding Move Requests

How can I change which AD account an Exchange mailbox is connected to without deleting the account?

Check if an emailis address is valid

Telnet

Detailed Guide On How To Do Mailbox Pinging

Use Telnet to test SMTP communication on Exchange servers

How to check an SMTP connection with a manual telnet session

How to check if an email address exists without sending an email?

Communicating with an email server using TELNET

How to Test SMTP Server from the Command Line via Telnet and in Online Tools

How Do I Ping an Email Address?

Resolving mail server problems with Nslookup and Telnet

NOT ABLE TO TELNET TO PUBLIC IP ADDRESS

What are Traceroute, Ping, Telnet and Nslookup commands?

SMTP Telnet Test

Setting the Source Address for Telnet

How to Send an SMTP Email

How to Test an Email Server with the Telnet Client

Cannot email or telnet on port 25 to external mail server…

Using Telnet To Check SMTP Service

How to Test SMTP Settings Using Telnet

Sending An Email Using Telnet

Testing for network connectivity

Test Your SMTP Mail Server via Telnet

SMTP Test via Telnet

How to test network connectivity with telnet

can’t telnet mail.mydomain.com!

Test SMTP connection

How to use Telnet to Send a Test Message and Verify SMTP Communication

How to check if an email address really exists

Troubleshooting with Telnet

System: Emulating an HTTP/1.1 Request using Telnet

How to Verify an Email Address?

How to Verify if an Email Address Is Real or Fake

How to check if an email address exists without sending an email?

How to check if an email address really exists

Command Line

PR HACK: HOW TO VERIFY AN EMAIL ADDRESS EXISTS

How to check whether an email address exists?

How to verify Email Addresses in Outlook before sending a message.

How to Verify If an Email Address Is Valid

How to check if an email address exists without sending an email?

How to Verify an Email Address

Offline Address Book (OAB)

Update-OfflineAddressBook‎ cmdlet missing

Global Address book note updating – exchange/Oultook 2010

The Offline Address Book (Part 4)

The Offline Address Book (Part 3)

The Offline Address Book (Part 2)

The Offline Address Book (Part 1)

How to Quickly Rebuild an Exchange 2010 Offline Address Book

Update-OfflineAddressBook

Updating the Offline Address Book in Exchange and Outlook

Connecting to Exchange Server

I want to remotely connect Sharepoint server through powershell whose database is on different machine

How to Manage PowerShell Remote Access with SharePoint Server 2013

How to connect to an Exchange server via PowerShell

Use PowerShell Remoting to Manage SQL Servers Efficiently

PowerShell Function to Connect to Exchange On-Premises

Run Powershell script remotely but not Exchange server

Connect to Exchange servers using remote PowerShell

Connecting to Office 365 using Powershell

How to start remote PowerShell session to Exchange or Office 365

How do I set up a remote PowerShell session with Exchange 2010+?

Connecting to Exchange 2010 with PowerShell

Connecting to Exchange Online with remote PowerShell from a Mac

Microsoft Exchange Server

Microsoft Exchange Server is a mail server and calendaring server developed by Microsoft. It runs exclusively on Windows Server operating systems.

Exchange Server was initially Microsoft’s internal mail server. The first version of Exchange Server to be published outside Microsoft was Exchange Server 4.0. Exchange initially used the X.400 directory service but switched to Active Directory later.

X.400 is a suite of ITU-T Recommendations that define standards for Data Communication Networks for Message Handling Systems (MHS) — more commonly known as email

The ITU Telecommunication Standardization Sector (ITU-T) is one of the three sectors (divisions or units) of the International Telecommunication Union (ITU); it coordinates standards for telecommunications.The standardization efforts of ITU commenced in 1865 with the formation of the International Telegraph Union (ITU). ITU became a Specialized agency of the United Nations in 1947. The International Telegraph and Telephone Consultative Committee (CCITT, from French: Comité Consultatif International Téléphonique et Télégraphique) was created in 1956, and was renamed ITU-T in 1993. ITU-T has a permanent secretariat, the Telecommunication Standardization Bureau (TSB), based at the ITU headquarters in Geneva, Switzerland. The current Director of the Bureau is Chaesub Lee, whose 4-year term commenced on 1 January 2015, who replaced Malcolm Johnson of the United Kingdom, who was director from 1 January 2007 to 2014.

Primary function of ITU-T:

The ITU-T mission is to ensure the efficient and timely production of standards covering all fields of telecommunications on a worldwide basis, as well as defining tariff and accounting principles for international telecommunication services.

The international standards that are produced by the ITU-T are referred to as “Recommendations” (with the word ordinarily capitalized to distinguish its meaning from the ordinary sense of the word “recommendation”), as they become mandatory only when adopted as part of a national law.

Since the ITU-T is part of the ITU, which is a United Nations specialized agency, its standards carry more formal international weight than those of most other standards development organizations that publish technical specifications of a similar form)

At one time, the designers of X.400 were expecting it to be the predominant form of email, but this role has been taken by the SMTP-based Internet e-mail. Despite this, it has been widely used within organizations and was a core part of Microsoft Exchange Server until 2006; variants continue to be important in military and aviation contexts ]

 Versions 4.0 and 5.0 came bundled with an email client called Microsoft Exchange Client. It was discontinued in favor of Microsoft Outlook.

Exchange Server uses a proprietary protocol called MAPI. Over time, however, it added support for POP3IMAPSMTP, and EAS

Post Office Protocol

POP – In computing, the Post Office Protocol (POP) is an application-layerInternet standard protocol used by local e-mail clients to retrieve e-mail from a remote server over a TCP/IP connection.[1] POP has been developed through several versions, with version 3 (POP3) being the last standard in common use before largely being made obsolete by the more advanced IMAP as well as webmail

Overview of POP

POP supports download-and-delete requirements for access to remote mailboxes (termed maildrop in the POP RFC‘s). Although most POP clients have an option to leave mail on server after download, e-mail clients using POP generally connect, retrieve all messages, store them on the user’s PC as new messages, delete them from the server, and then disconnect. Other protocols, notably IMAP, (Internet Message Access Protocol) provide more complete and complex remote access to typical mailbox operations. In the late 1990s and early 2000s, fewer Internet Service Providers (ISPs) supported IMAP due to the storage space that was required on the ISP’s hardware. Contemporary e-mail clients supported POP, then over time popular mail client software added IMAP support.

A POP3 server listens on well-known port 110. Encrypted communication for POP3 is either requested after protocol initiation, using the STLS command, if supported, or by POP3S, which connects to the server using Transport Layer Security (TLS) or Secure Sockets Layer (SSL) on well-known TCP port 995.

Available messages to the client are fixed when a POP session opens the maildrop, and are identified by message-number local to that session or, optionally, by a unique identifier assigned to the message by the POP server. This unique identifier is permanent and unique to the maildrop and allows a client to access the same message in different POP sessions. Mail is retrieved and marked for deletion by message-number. When the client exits the session, the mail marked for deletion is removed from the maildrop.

Internet Message Access Protocol 

IMAP – In computing, the Internet Message Access Protocol (IMAP) is an Internet standard protocol used by e-mail clients to retrieve e-mail messages from a mail server over a TCP/IP connection. IMAP is defined by RFC 3501.IMAP was designed with the goal of permitting complete management of an email box by multiple email clients, therefore clients generally leave messages on the server until the user explicitly deletes them. An IMAP server typically listens on port number 143. IMAP over SSL (IMAPS) is assigned the port number 993.Virtually all modern e-mail clients and servers support IMAP. IMAP and the earlier POP3 (Post Office Protocol) are the two most prevalent standard protocols for email retrieval,[2] with many webmail service providers such as GmailOutlook.com and Yahoo! Mail also providing support for either IMAP or POP3.

E-mail protocols of IMAP

The Internet Message Access Protocol is an Application Layer Internet protocol that allows an e-mail client to access e-mail on a remote mail server. The current version, IMAP version 4 revision 1 (IMAP4rev1), is defined by RFC 3501. An IMAP server typically listens on well-known port 143. IMAP over SSL (IMAPS) is assigned well-known port number 993.

IMAP supports both on-line and off-line modes of operation. E-mail clients using IMAP generally leave messages on the server until the user explicitly deletes them. This and other characteristics of IMAP operation allow multiple clients to manage the same mailbox. Most e-mail clients support IMAP in addition to Post Office Protocol (POP) to retrieve messages; however, fewer e-mail services support IMAP.[3] IMAP offers access to the mail storage. Clients may store local copies of the messages, but these are considered to be a temporary cache.

Incoming e-mail messages are sent to an e-mail server that stores messages in the recipient’s e-mail box. The user retrieves the messages with an e-mail client that uses one of a number of e-mail retrieval protocols. Some clients and servers preferentially use vendor-specific, proprietary protocols, but most support SMTP for sending e-mail and POP and IMAP for retrieving e-mail, allowing interoperability with other servers and clients. For example, Microsoft‘s Outlook client uses MAPI, a Microsoft proprietary protocol, to communicate with a Microsoft Exchange ServerIBM‘s Notes client works in a similar fashion when communicating with a Domino server. All of these products also support POP, IMAP, and outgoing SMTP. Support for the Internet standard protocols[citation needed] allows many e-mail clients such as Pegasus Mail or Mozilla Thunderbird to access these servers, and allows the clients to be used with other servers.

Simple Mail Transfer Protocol 

SMTP – Simple Mail Transfer Protocol (SMTP) is an Internet standard for electronic mail (email) transmission. First defined by RFC 821 in 1982, it was last updated in 2008 with Extended SMTP additions by RFC 5321, which is the protocol in widespread use today.Although electronic mail servers and other mail transfer agents use SMTP to send and receive mail messages, user-level client mail applications typically use SMTP only for sending messages to a mail server for relaying. For retrieving messages, client applications usually use either IMAP or POP3.SMTP communication between mail servers uses port 25. Mail clients on the other hand, often submit the outgoing emails to a mail server on port 587. Despite being deprecated, mail providers sometimes still permit the use of nonstandard port 465 for this purpose.SMTP connections secured by SSL, known as SMTPS, can be made using STARTTLS.[1]Although proprietary systems (such as Microsoft Exchange and IBM Notes) and webmail systems (such as Outlook.comGmail and Yahoo! Mail) use their own non-standard protocols to access mail box accounts on their own mail servers, all use SMTP when sending or receiving email from outside their own systems.

Mail processing model of SMTP

Email is submitted by a mail client (mail user agent, MUA [also known as Email Client e.g Outlook 2010]) to a mail server (mail submission agent, MSA [A message submission agent (MSA) or mail submission agent is a computer program or software agent that receives electronic mail messages from a mail user agent (MUA) and cooperates with a mail transfer agent (MTA) for delivery of the mail. It uses ESMTP, a variant of the Simple Mail Transfer Protocol (SMTP), as specified in RFC 6409.[1]Many MTAs perform the function of an MSA as well, but there are also programs that are specially designed as MSAs without full MTA functionality. Historically, in Internet mail, both MTA and MSA functions use port number 25, but the official port for MSAs is 587.[1] The MTA accepts incoming mail, while the MSA accepts outgoing mail.]) using SMTP on TCP port 587. Most mailbox providers still allow submission on traditional port 25. 

The MSA delivers the mail to its mail transfer agent (mail transfer agent, MTA). Often, these two agents are instances of the same software launched with different options on the same machine. Local processing can be done either on a single machine, or split among multiple machines; mail agent processes on one machine can share files, but if processing is on multiple machines, they transfer messages between each other using SMTP, where each machine is configured to use the next machine as a smart host. Each process is an MTA (an SMTP server) in its own right. 

The boundary MTA uses the Domain name system (DNS) to look up the mail exchanger record (MX record) for the recipient’s domain (the part of the email address on the right of @). The MX record contains the name of the target host. Based on the target host and other factors, the MTA selects an exchange server: see the article MX record. The MTA connects to the exchange server as an SMTP client.

Message transfer can occur in a single connection between two MTAs, or in a series of hops through intermediary systems. A receiving SMTP server may be the ultimate destination, an intermediate “relay” (that is, it stores and forwards the message) or a “gateway” (that is, it may forward the message using some protocol other than SMTP). Each hop is a formal handoff of responsibility for the message, whereby the receiving server must either deliver the message or properly report the failure to do so.[15]

Once the final hop accepts the incoming message, it hands it to a mail delivery agent (MDA) for local delivery. An MDA saves messages in the relevant mailbox format. As with sending, this reception can be done using one or multiple computers, but in the diagram above the MDA is depicted as one box near the mail exchanger box. An MDA may deliver messages directly to storage, or forward them over a network using SMTP or other protocol such as Local Mail Transfer Protocol (LMTP), a derivative of SMTP designed for this purpose.

Once delivered to the local mail server, the mail is stored for batch retrieval by authenticated mail clients (MUAs). Mail is retrieved by end-user applications, called email clients, using Internet Message Access Protocol (IMAP), a protocol that both facilitates access to mail and manages stored mail, or the Post Office Protocol (POP) which typically uses the traditional mbox mail file format or a proprietary system such as Microsoft Exchange/Outlook or Lotus Notes/DominoWebmail clients may use either method, but the retrieval protocol is often not a formal standard.

SMTP defines message transport, not the message content. Thus, it defines the mail envelope and its parameters, such as the envelope sender, but not the header (except trace information) nor the body of the message itself. STD 10 and RFC 5321 define SMTP (the envelope), while STD 11 and RFC 5322 define the message (header and body), formally referred to as the Internet Message Format.

Exchange ActiveSync 

EAS – Exchange ActiveSync (commonly known as EAS) is a communications protocol designed for the synchronization of email, contacts, calendar, tasks, and notes from a messaging server to a smartphone or other mobile devices. The protocol also provides mobile device management and policy controls. The protocol is based on XML. The mobile device communicates over HTTP or HTTPS. Originally branded as AirSync and only supporting Microsoft Exchange Servers and Pocket PC devices, the protocol has since become a de facto standard for synchronization between groupware and mobile devices.Microsoft licenses the technology. Support for EAS is now implemented in a number of competing collaboration platforms, including GroupWise with the Novell GroupWise Mobility Services software and Lotus Notes with IBM Notes TravelerGoogle previously offered support for the protocol for personal Gmail and free Google Apps accounts, but began removing support from all but paid Google Apps for Work subscriptions in 2013. Beyond on premises installations of Exchange, the various personal and enterprise hosted services from Microsoft also utilize EAS, including Outlook.com and Office 365.In addition to support on Windows Phone, EAS client support is included on AndroidiOSBlackBerry 10 smartphones and the BlackBerry PlayBook tablet computer. The built-in email application for Windows 8 desktop, Mail app, also supports the protocol.

Exchange Server is licensed both in the forms of on-premises software and software as a service. In the on-premises form, customer purchase client access licenses (CALs). In the software as a service form, Microsoft receives a monthly service fee instead (see Microsoft Office 365). 

References:

Microsoft Exchange Server

X.400

ITU-T

Post Office Protocol

Internet Message Access Protocol

Simple Mail Transfer Protocol

Message Submission Agent

Email Client

Exchange ActiveSync