invoke appropriate MTA software based
on configuration file
Special. See below.
At one time, the only Mail Transfer Agent (MTA) software easily available was sendmail(8). As a result of this, most Mail User Agents (MUAs) such as mail(1) had the path and calling conventions expected by sendmail(8) compiled in.
Times have changed, however. On a modern 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(8) and newaliases(8) linked to it. The program knows to behave differently when its argv 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 drop-in replacements for sendmail(8) helps in installing alternative MTAs, it essentially makes the configuration of the system depend on hard 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.
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.
mailwrapper is kept in
/usr/sbin/sendmail is typically set up as a symlink
mailwrapper which is not usually invoked on its
mailwrapper will invoke
smtpd(8) if its configuration file is missing. It will return an
error value and print a diagnostic if its configuration file is malformed,
or does not contain a mapping for the name under which
mailwrapper was invoked.
mail(1), mailer.conf(5), mailq(8), newaliases(8), sendmail(8), smtpd(8)
Perry E. Metzger <email@example.com>
The entire reason this program exists is a crock. Instead, a command for how to submit mail should be standardized, and all the "behave differently if invoked with a different name" behavior of things like mailq(8) should go away.