Email Protocols

Explain how email protocols operate.

• Email is a store-and-forward method of sending, storing, and retrieving electronic messages across a network.
• Email messages are stored in databases on mail servers.
• Email clients communicate with mail servers to send and receive email.
• The email protocols used for operation are:
  • Simple Mail Transfer Protocol (SMTP) – used to send mail.
  • Post Office Protocol (POP) & IMAP – used for clients to receive mail.

SMTP, POP and IMAP

• When a client sends email, the client SMTP process connects with a server SMTP process on well-known port 25.
• After the connection is made, the client attempts to send the email to the server across the connection.
• When the server receives the message, it either places the message in a local account, if the recipient is local, or forwards the message to another mail server for delivery.
• The destination email server may not be online or may be busy. If so, SMTP spools messages to be sent at a later time.

  Note: SMTP message formats require a message header (recipient email address & sender email address) and a message body.

• POP is used by an application to retrieve mail from a mail server. When mail is downloaded from the server to the client using POP the messages are then deleted on the server.
  • The server starts the POP service by passively listening on TCP port 110 for client connection requests.
  • When a client wants to make use of the service, it sends a request to establish a TCP connection with the server.
  • When the connection is established, the POP server sends a greeting.
  • The client and POP server then exchange commands and responses until the connection is closed or aborted.

Note: Since POP does not store messages, it is not recommended for small businesses that need a centralized backup solution.

• IMAP is another protocol that describes a method to retrieve email messages.
  • Unlike POP, when a user connects to an IMAP server, copies of the messages are downloaded to the client application. The original messages are kept on the server until manually deleted.
  • When a user decides to delete a message, the server synchronizes that action and deletes the message from the server.

Other related topics

Other useful information

• Full CCNA Course
• CCNA Certificate Information
• 200-301 CCNA Exam Questions and Solutions
• 200-301 CCNA Exam Topics