Actually SAPD is- A SIMPLE E-MAIL SECURITY DAEMON
The protocol used to send a-mail, the SMTP, has a severe flaw, that is
the fact it doensn't require passwords. Due to this fact, the e-mail
server is vulnerable to all kinds of abuse via the Internet, like SPAM
relaying and origination of e-mail with the sender's address forged.
SAPD is a daemon designed to protect a Sendmail server against such
abuses, demanding the user fetches (reads) his e-mails before he can
start sending. As fetching e-mail requires an username and password,
we have here a good authentication method that prevents misuse of the
This kind of authentication is called SMTP-After-POP3, because it
allows sending e-mail (SMTP) only after fetching e-mail (POP3). Hence
the name SAPD, that means "SMTP After POP3 Daemon".
For more information : Documentation