What is a mail delivery subsystem

A mail delivery subsystem is a component of a larger email system that is responsible for delivering email messages from a mail transfer agent (MTA) to a mail delivery agent (MDA) on a recipient's email server. The mail delivery subsystem is typically responsible for the final stage of email delivery, which is the transfer of the email message from the MTA to the MDA on the recipient's email server.

The mail delivery subsystem typically performs the following functions:

  1. Message filtering: The mail delivery subsystem may apply filters to the email message to determine whether it should be delivered to the recipient's mailbox or rejected due to spam, viruses, or other issues.
  2. Message routing: The mail delivery subsystem determines the correct route for the email message to take to reach the recipient's email server. This may involve looking up the recipient's email address in a database or using DNS (Domain Name System) to determine the IP address of the recipient's email server.
  3. Message delivery: The mail delivery subsystem transfers the email message to the recipient's email server using a protocol such as SMTP (Simple Mail Transfer Protocol) or LMTP (Local Mail Transfer Protocol).
  4. Message storage: The mail delivery subsystem may store the email message in the recipient's mailbox on the email server.

The mail delivery subsystem is typically implemented as a software component on the email server, and may be integrated with other email system components such as the MTA, MDA, and email client software.

Some common examples of mail delivery subsystems include:

  1. Postfix: A popular open-source MTA that includes a mail delivery subsystem.
  2. Sendmail: A widely-used MTA that includes a mail delivery subsystem.
  3. Exim: A popular open-source MTA that includes a mail delivery subsystem.
  4. Microsoft Exchange: A commercial email server software that includes a mail delivery subsystem.
  5. IBM Domino: A commercial email server software that includes a mail delivery subsystem.

In summary, the mail delivery subsystem is a critical component of an email system that is responsible for delivering email messages to the recipient's email server, and is typically implemented as a software component on the email server.