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26.1 Send and receive electronic mail via SMTP, sendmail

When a mail program such as mail tries to send a message it issues a request to sendmail, which processes the mail with the specified options. sendmail creates a list of recipients from the information and expands any aliases, including mailing lists. At this step syntax is checked and local addresses are verified. Duplicate recipients are removed, e.g. the same person being a member of two groups. If no addresses are valid the message is returned with an error message. sendmail then tries to deliver the message. If it can't deliver the message immediately it stores the header and body of the message in temporary files in a queue (/var/spool/mqueue) and tries to send it again latter.

SMTP stands for Simple Mail Transfer Protocol, and is the protocol used for Internet mail. It requires an entry in /etc/services, i.e.:

smtp 25/tcp mail

You can telnet to the mail port to see how you server is responding, i.e.:

# telnet localhost 25

It should respond with the fully qualified domain name (fqdn), otherwise, your machine may have trouble communicating with other mail servers.

Unix System Administration - 8 AUG 1996
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