Mailbox Servers

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• SMTP is used only to deliver mail to server hosts that are permanently available
• When an email is received by an SMTP server, it delivers the message to a mailbox server
  • could be a separate machine or a separate process running on the same computer
• mailbox access protocol allows the user’s client email software to retrieve messages from the mailbox

Post Office Protocol 3 (POP3)

Post office protocol (POP) is an Internet standard protocol used by an email client to retrieve an email from a mail server.

• does not support sending of an email
• how it works
  • a mailbox/folder is created for an email account on a POP server
  • when email is received, it is saved to the recipient’s mailbox
  • email account owner uses a POP client to connect to a POP server and download the account owner’s email
  • email is removed from the mailbox after it is downloaded by a POP client
    • by default

Post Office Protocol v3 (POP3) is an application protocol that enables a client to download email messages from a server mailbox to a client over port TCP/110 or secure port TCP/995.

• is an early example of a mailbox access protocol
  • POP3 because the active version of the protocol is version 3
• provides confidentiality and integrity

How it Works

• A POP client application establishes a connection to the POP server on port TCP/110 or over the secure port TCP/995 (POP3S)
  • clients such as Microsoft Outlook® or Mozilla Thunderbird®
• user is authenticated by username and password
• the contents of the mailbox are downloaded for processing on the local PC
  • With POP3, the messages are typically deleted from the mailbox server when they are downloaded
    • some clients have the option to leave messages on the server

Internet Message Access Protocol (IMAP)

Internet Message Access Protocol (IMAP) is an application protocol providing a means for a client to access and manage email messages stored in a mailbox on a remote server.

• addresses some of the limitations of POP
• is a mail retrieval protocol, but its mailbox management features lack the features associated with POP mail management
• supports:
  • permanent connections to a server
  • and connecting multiple clients to the same mailbox simultaneously
  • allows a client to manage the mailbox on the server and to create multiple mailboxes
• by default, email remains on the IMAP server after downloaded by an IMAP client
  • only removed from an IMAP server if user explicitly deletes it on the IMAP server
• client connects to an IMAP server over port TCP/143
  • port is unsecure
  • clients authenticate then retrieve messages from the designated folders/mailboxes
• Connection security can be established using TLS
  • default port for IMAP-Secure (IMAPS) is TCP/993