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 Unix 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 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 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.
The mailwrapper utility is designed to replace /usr/sbin/sendmail and to invoke an appropriate MTA instead of sendmail(8) based on configuration information placed in /etc/mail/mailer.conf. This permits the administrator to configure which MTA is to be invoked on the system at run time.
Other configuration files may need to be altered when replacing sendmail(8). For example, if the replacement MTA does not support the -A option with mailq(1), daily_status_include_submit_mailq should be turned off in /etc/periodic.conf.
Configuration for mailwrapper is kept in /etc/mail/mailer.conf. /usr/sbin/sendmail is typically set up as a symbolic link to mailwrapper which is not usually invoked on its own.
The mailwrapper utility will return an error value and print a diagnostic if its configuration file is missing or malformed, or does not contain a mapping for the name under which mailwrapper was invoked.
mail(1), mailq(1), newaliases(1), mailer.conf(5), periodic.conf(5), sendmail(8)