The mail message is scanned for lines beginning `From:', `To:' and `Cc:'.
If no `from' line is found, the sender is assumed to be the current user. The recipient(s) of the message can be mentioned as arguments or in a list of names in a `To:' line but not as both. If the sender's name is unqualified (is just a user name), sendmail appends the value of the environment variable DOMAIN.

Sendmail delivers the mail using the module smtp(2), which connects to a mail server running the Simple Mail Transport Protocol (SMTP), using the symbolic host name $MAILSERVER (see db(6)).