Usually, there is only one
MTA on a machine. The sendmail program is an MTA.
Beginning withV8.10, sendmail also recognizes the role of a mail submission agent
(MSA), as defined in RFC2476. MTAs are not supposed to alter an email??™s text,
except to add Received:, Return-Path:, and other required headers. Email submitted
by an MUA might require more modification than is legal for an MTA to perform, so
the new role of an MSA was created. An MSA accepts messages from an MUA, and
has the legal right to heavily add to, subtract from, and screen or alter all such email.
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1.3 Email and sendmail | 3
An MSA, for example, can ensure that all hostnames are fully qualified, and that
headers, such as Date:, are always included.
1.3.1 Other MTAs
The sendmail program is not the only MTA on the block. Others have existed for
some time, and new MTAs appear on the scene every once in a while. Here we
describe a few of the other major MTAs:
qmail
Stressing modularity and security, qmail claims to be a replacement for sendmail.
Th eqmail program is an open source offering, available from http://
www.qmail.org.
Postfix
Written by Wietse Venema, a security expert on the IBM Research staff, Postfix
is advertised to be a drop-in replacement for sendmail that purports to deliver
email more quickly, conveniently, and safely.
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