Times have changed, however. On a modern NetBSD system, the administrator may wish to use one of several available MTAs.
It would be difficult to modify all MUA software typically available
on a system, so most of the authors of alternative MTAs have written
their front end message submission programs so that they use the same
calling conventions as
sendmail(8)
and may be put into place instead of
sendmail(8)
in
/usr/sbin/sendmail
.
sendmail(8) also typically has aliases named mailq(1) and newaliases(1) linked to it. The program knows to behave differently when its argv[0] is ``mailq'' or ``newaliases'' and behaves appropriately. Typically, replacement MTAs provide similar functionality, either through a program that also switches behavior based on calling name, or through a set of programs that provide similar functionality.
Although having replacement programs that plug replace
sendmail(8)
helps in installing alternative MTAs, it essentially makes the
configuration of the system depend on hand installing new programs in
/usr
.
This leads to configuration problems for many administrators, since
they may wish to install a new MTA without altering the system
provided
/usr
.
(This may be, for example, to avoid having upgrade problems when a new
version of the system is installed over the old.)
They may also have a shared
/usr
among several
machines, and may wish to avoid placing implicit configuration
information in a read-only
/usr
.
The
mailwrapper
program is designed to replace
/usr/sbin/sendmail
and to invoke an appropriate MTA instead of
sendmail(8)
based on configuration information placed in
/etc/mailer.conf
.
This permits the administrator to configure which MTA is to be invoked on
the system at run time.
/etc/mailer.conf
.
/usr/sbin/sendmail
is typically set up as a symlink to
mailwrapper
which is not usually invoked on its own.