TCP/IP Protocol: Electronic Mail (E-Mail) Protocols


Jarret W. Buse

TCP/IP Protocol: Electronic Mail (E-Mail) Protocols

There are three primary TCP/IP protocols for E-Mail management:

  • Post Office Protocol (POP)
  • Simple Mail Transfer Protocol (SMTP)
  • Internet Message Access Protocol (IMAP)
Post Office Protocol (POP)

POP is an Application Layer protocol for a TCP/IP network for the transferring of e-mail. The current version of POP is version 3 (POP3) which uses port 110. Encrypted POP3, usually referred to as POP3S, uses port 995 for Transport Layer Security (TLS) or Secure Sockets Layer (SSL). POP3 is usually used for incoming mail from the server, such as Hotmail, and to access the client mailbox.

Internet Message Access Protocol (IMAP)

IMAP is also an Application Layer protocol. IMAP is currently at version 4 revision 1 (IMAP4rev1) and operates on a TCP/IP network on port 143. Secure IMAP uses Secure Sockets Layer (SSL) uses port 993.

IMAP has previously been known as Interim Mail Access Protocol, Internet Mail Access Protocol, and Interactive Mail Access Protocol.

There are some differences between POP and IMAP:

  • POP allows only one client to connect to a mailbox at once, IMAP allows multiple clients that share a mailbox to connect simultaneously
  • IMAP4 allows clients to download any part of the MIME message. The portion can be the header, message or attachments
  • POP allows a client to connect to a server long enough to download messages. IMAP can keep a client connected even when messages are not being transferred so when a client does transfer a message, there is no time required to reconnect to the server
  • IMAP4 allows for Access Control Lists (ACL) to be created to set permissions on multiple mailboxes for a single client
  • IMAP4 can send a query to the server to allow the server to search through all messages in a mailbox. This prevents a client from downloading all messages to perform the search locally
  • IMAP4 allows the setting of tags on messages to represent if the message has been read, deleted or replied
  • POP3 and IMAP4 have extension mechanisms which allow for extensions to be created for extra functionality
Simple Mail Transfer Protocol (SMTP)

SMTP is an Application Layer protocol for a TCP/IP network for the transferring of e-mail. SMTP uses port 25 or possibly 587 in some configurations. SMTP with Secure Sockets Layer (SSL), known as SMTPS uses port 465. SMTP is typically used for outgoing e-mail to a server for delivery to other clients, whether the server is local or not.

Once a client connects to the E-mail Server, there may be 0 or more SMTP transactions. If the client has no mail to send, then there are no SMTP transactions. Every e-mail message sent is an SMTP transfer. The transfer consists of three commands and replies:

  1. MAIL – sets up return address to client system
  2. RCPT – sets up the recipient of the message
  3. DATA – the message portion of the e-mail
NOTE: The e-mail server will respond twice to the DATA command. Initially, the response is to verify to the client that it is ready to receive the message and the second when the end-of-data marker is sent. Once the second response is sent, the client knows the server received the message.

Keep in mind that SMTP is only used to send (push) messages to the server. POP and IMAP are used to receive messages as well as manage the mailbox contents. Mailbox management consists of performing tasks on messages such as deleting, moving, etc.

Server Software

Postfix and Dovecot are two e-mail server software applications for Linux. Postfix is an SMTP server application which can use Dovecot to implement POP3 and IMAP4.

NOTE: Be aware, these are only example applications. Other applications exist which may suit your needs better.

The procedure is to remove any mail services from your Linux system so there are no conflicting applications. Install Postfix and configure as needed by the documentation. Check that Postfix is operational after installation and configuration. Once Postfix is working, install and configure Dovecot. Test Dovecot to verify the installation and configuration. At this point, you may want to install and configure SquirrelMail. Once done, restart the Linux system to allow all services to start. SquirrelMail allows clients to access their mailboxes through an HTTP page and not require an e-mail client application, only a web browser.

For the home pages of the above applications go to:
  • Postfix –
  • Dovecot –
  • SquirrelMail –

Client Software

Linux has numerous e-mail client applications. One good application is Thunderbird made by Mozilla.

NOTE: Other e-mail client software exists which may be more suited for your specific needs. Check the functionality of other applications to determine which work best for you.

Thunderbird can be found at

Happy e-mailing!


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