2003/10/02 SMTP

Simple Mail Transport Protocol is how the internet transports mail from computer to computer. It's a very simple, text based conversation between two computers that might look something like this:

 220 smtp.somewhere.net ESMTP Sendmail 8.9.3+Sun/8.9.1; Thu, 12 Oct 2000 04:39:40 -0700 (PDT)
 mail from: tony@aplawrence.com
 250 tony@aplawrence.com... Sender ok
 rcpt to: foobah@somewhere.com
 250 foobah@somewhere.com... Recipient ok
 data
 354 Enter mail, end with . on a line by itself
 test
 look ma no headers!
 .
 250 HA00945 Message accepted for delivery
 quit
 221 smtp.somewhere.com closing connection
 

Servers use SMTP to both send and receive mail. Your email client (Outlook, Eudora, whatever) uses SMTP to send mail, but does NOT receive its mail from other computers this way. Clients will use protocols like POP or IMAP to receive mail.

When servers send mail, they talk directly to other systems: if mail goes from aol.com to me, AOL's computer's make a connection to my server. Your desktop client doesn't do that: it needs an intermediary, a "default smtp server" that it will send to. That server will make the actual connection to me. The reason for this is that Outlook and other email clients can't be bothered with the problems of keeping track of mail that can't be delivered right now - they worry about the default server only. My server may be down or very busy just now and not ready to accept your mail. Desktop email clients don't want the complexity of dealing with all that, so they pass that responsibility to the intermediary. That server may have to temporarily hold mail for me, for joe and whoever, but that is its job, and whenever my server and joe's are available again, it will send the mail.

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