Delivery agents are declared using the MAILER directive:
The available agents are listed in the cf/mailer directory (see §19.3.2). For example, consider:
Here, the first includes support for local delivery, delivery to files, and delivery through programs. The second allows the sendmail daemon to accept incoming mail.
cyrus for delivery to the Cyrus IMAP4 server,
cyrusbb for delivery to a Cyrus bulletin board.
fax for delivery to a mail-to-fax gateway.
local for delivery to a local mailbox
prog for delivery through a shell program.
mail11 for delivery to a DECnet/mail11 gateway.
uses the phquery program to send to the ph directory service.
pop for delivery through the MH spop program.
procmail for delivery using the procmail program.
smtp, which handles SMTP mail;
esmtp, which always speaks ESMTP instead of adapting
on the basis of the greeting message;
smtp8, which always uses sends-8-bit data directly;
relay, which uses SMTP with minimal rewriting
and only works within a single domain.
usenet for delivery to a mail-to-news gateway.
uucp-old if your peers are running very old
(V7) versions of UUCP.
uucp-new if they are running a newer version
(e.g., honey danber) but don't understand domain-based addressing.
uucp-uudom if your peers use UUCP addresses
(e.g., host!user) in the envelope, but domain-based addresses
(email@example.com) in the headers.
uucp-dom if your peers are completely
converted to domain-based addressing.
The last two are only defined if MAILER(smtp) is declared