From the course: Linux System Engineer: Mail Systems Using Postfix

About email

From the course: Linux System Engineer: Mail Systems Using Postfix

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About email

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- [Instructor] Email was the killer application that drove the creation of the internet as we know it. It's still the most popular application on the internet, although now we have many other ways of communicating. Electronic mail works works in a similar way to snail mail in that we send letters or messages from one person to another, which are delivered by multiple carriers between the two. Let's cover some terms associated with email systems. With email, the protocol that sends from one email server to another is SMTP, or simple mail transfer protocol. The SMTP protocol runs on IP for host addressing and uses port number 25. Just as a traditional letter has a sender and destination address, email has a sender's ID and a domain along with a destination email address. It also carries a subject line in the message itself. The email system comprises of a mail user agent, or MUA, which is the program that sends the mail from the user's computer to a mail transport agent, or MTA. A mail transport agent, or MTA, is responsible for delivering the email from the sending mail server to the ending mail server. A mail submission agent, or MSA, is responsible for accepting new mail for the MTA. This functionality may be included in the MTA or MUA software applications or it may be a separate program. A mail delivery agent, or MDA, is responsible for delivering incoming messages to the mail spool. This also may be integrated in the MTA, or it may be a separate program. We've already discussed the SMTP protocol used by mail user agents to send mail to MTAs. It's also used between MTAs, however for downloading mail from the MTA, there are two common protocols used by the mail user agent. The first is post office protocol, or POP. The last version of POP was POP3 and it uses port number 110. By default, POP deletes the email from the server when you download it, although there are some POP clients that will optionally leave the mail there. It's also possible to use a secure version of POP called POP3s, which uses SSL and port number 995. The other protocol for downloading email is internet mail access protocol, or IMAP. IMAP is more powerful than POP in that it's easier to access mail using multiple systems and works better over slow connections. Because IMAP only downloads a copy of the email, a user can login to a different host and retrieve their email again. A smart host or a mail relay is an MTA configured to deliver mail for another system. The other system may not have a constant connection to the internet, so it delivers mail to the relay, which then sends it on. A mail queue is a directory where mail is stored while it's waiting to be processed. And lastly, a mailbox is a place where user mail is stored. A mailbox is created for each user.

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