simple mail transfer protocol daemon
smtpd
The smtpd daemon takes all of the sendmail options except bi and bp.
The smtpd daemon is responsible for listening on a host's SMTP port and dealing with the mail it receives as defined in the sendmail.cf configuration file.
Note that smtpd is identical to the command:
sendmail -bd
sendmail,
/etc/sendmail.cf file
Sendmail by Bryan Costales, O'Reilly & Associates (ISBN 1-56592-056-2)