Another role of sendmail is that of transporting mail to other machines. A message is transported when sendmail determines that the recipient is not local. The following lines from a typical configuration file define delivery agents for transporting mail to other machines:
Msmtp, P=[IPC], F=mDFMuX, S=11/31, R=21, E=\r\n, L=990, Muucp, P=/usr/bin/uux, F=DFMhuUd, S=12, R=22/42, M=10000000,
The actual lines in your file may differ. The name
Msmtp, P=[IPC], F=mDFMuX, S=11/31, R=21, E=\r\n, L=990,
When sendmail transports mail on a TCP/IP network, it first sends the envelope-sender hostname to the other site. If the other site accepts the sender's hostname as legal, the local sendmail then sends the envelope-recipient list. The other site accepts or rejects each recipient one by one. If any recipients are accepted, the local sendmail sends the message (header and body together).
Muucp, P=/usr/bin/uux, F=DFMhuUd, S=12, R=22/42, M=10000000,
This line tells sendmail to send UUCP network mail by running the /bin/uux ( UNIX to UNIX eXecute ) program.
There are many other kinds of network protocols that sendmail can use to transport email. Some of them may have shown up when you ran grep earlier. Other common possibilities might look like one of these:
Mfax, P=/usr/local/lib/fax/mailfax, F=DFMhu, S=14, R=24, M=100000, Mmail11, P=/usr/etc/mail11, F=nsFx, S=15, R=25, A=mail11 $g $x $h $u Mmac, P=/usr/bin/macmail, F=CDFMmpsu, R=16, S=16, A=macmail -t $u
In all of these examples, note that sendmail sends email over other networks by running programs that are tailored specifically for that use. Remember that the only network that sendmail can use directly is a TCP/IP-based network.