Incoming mail

sendmail processes and delivers incoming mail the same way it processes and delivers outgoing mail; the only difference being the source of the mail. sendmail receives incoming mail from other hosts via servers. The standard sendmail configuration supports two servers: SMTP and UUCP.

SMTP server

One instance of sendmail, generally invoked by the /etc/rc2.d/P86sendmail script at boot time, runs as the SMTP server by default. This server listens on the SMTP (Simple Mail Transfer Protocol) port for SMTP requests. When it receives a request, the sendmail daemon executes the fork system call to start a new instance of sendmail to process the request.

UUCP server

A standard SCO distribution includes a UUCP mail server called rmail(ADMN).
This standard program routes incoming mail from UUCP to sendmail by executing a new instance of sendmail.