Sendmail sends a message to one or more recipients, rout-
ing the message over whatever networks are necessary.
Sendmail does internetwork forwarding as necessary to
deliver the message to the correct place.

Sendmail is not intended as a user interface routine;
other programs provide user-friendly front ends; sendmail
is used only to deliver pre-formatted messages.

With no flags, sendmail reads its standard input up to an
end-of-file or a line consisting only of a single dot and
sends a copy of the message found there to all of the
addresses listed. It determines the network(s) to use
based on the syntax and contents of the addresses.

Local addresses are looked up in a file and aliased appro-
priately. Aliasing can be prevented by preceding the
address with a backslash. Beginning with 8.10, the sender
is included in any alias expansions, e.g., if `john' sends
to `group', and `group' includes `john' in the expansion,
then the letter will also be delivered to `john'.

Parameters
-Ac Use submit.cf even if the operation mode does not
indicate an initial mail submission.

-Am Use sendmail.cf even if the operation mode indi-
cates an initial mail submission.

-Btype Set the body type to type. Current legal values
are 7BIT or 8BITMIME.

-ba Go into ARPANET mode. All input lines must end
with a CR-LF, and all messages will be generated
with a CR-LF at the end. Also, the ``From:'' and
``Sender:'' fields are examined for the name of the
sender.

-bd Run as a daemon. Sendmail will fork and run in
background listening on socket 25 for incoming SMTP
connections. This is normally run from /etc/rc.

-bd之后就监听25, 变成smtp server,默认就是

sends a message to one or more recipients, rout-
ing the message over whatever networks are necessary.
sendmail本身就是用来发信的，不需要其它软件支持了