The rmail program is the dispatcher part of the UUCP suite of software. UUCP is an old-style means of moving email between machines that were only connected with dial-up modems. Although UUCP has almost entirely evaporated from most of the world, it still remains useful. The rmail program is a restricted form of the /bin/mail program that also understands UUCP routing.
For those rare sites that still run UUCP, sendmail offers a replacement for the frequently broken rmail program. The source in the rmail directory is not suitable for all operating systems. It is the original 4.4BSD source and will work as is only on 4.4BSD-based systems. It is included in the sendmail distribution as a starting point for porting to other versions of Unix.
We won’t detail how to build and use the rmail program here. If you need it, you should know why you do and, therefore, should have some ideas about how to port it. If you don’t run UUCP, or if you do and have no problems, you can skip rmail.