What is a Mail Server?

A mail server is the computerized equivalent of your friendly neighborhood mailman. Every email that is sent passes through a series of mail servers along its way to its intended recipient.

Although it may seem like a message is sent instantly – zipping from one PC to another in the blink of an eye – the reality is that a complex series of transfers takes place.

Without this series of mail servers, you would only be able to send emails to people whose email address domains matched your own – i.e., you could only send messages from one example.com account to another example.com account.

An email server is a computer with mail transfer agent (MTA) functions. Mail is exchanged between email servers running special software, which is built around standardized protocols for handling messages and their varied (multimedia) content.

An email server receives mail from another MTA, a mail user agent (MUA) or a mail submission agent (MSA) with the details of the transmission being defined by SMTP.

When an MTA receives a mail and the recipient of the mail is not hosted locally, the mail is forwarded to another MTA. Every time this happens the MTA adds a “received” trace header on the top header of the message.

This shows all the MTAs that have handled the message before it arrived in the recipient’s inbox. This useful feature allows administrators to see whether an optimal path was taken.