Role in TCP/IP
The sendmail program has the internal ability to transport mail over only one kind of network, one that uses TCP/IP; the following line instructs sendmail to do this:
Msmtp, P=[IPC], F=mDFMuX, S=EnvFromSMTP/HdrFromSMTP, R=EnvToSMTP/HdrFromSMTP,
The [IPC] might appear as
[TCP], but note
that, beginning with V8.10 sendmail,
the expression [TCP] is
deprecated, and it has been dropped entirely in
V8.12.
When sendmail transports mail on a TCP/IP network, it first sends the envelope-sender’s address to the other site. If the other site accepts the sender’s address as legal, the local sendmail then sends the list of envelope-recipient addresses. The other site accepts or rejects each recipient address one by one. If any recipient addresses are accepted, the local sendmail sends the message (header and body together). This kind of transaction for sending email is called SMTP and is defined in RFC2821.
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