BABYL is the name of an e-mail package created under ITS at the MIT AI Lab, written in TECO. It was a replacement for an earlier package called RMAIL. BABYL is still available under ITS and TOPS-20 (a DEC development of the TENEX operating system) both on real hardware and under the KLH10 and SimH? emulators of the PDP-10 processor; the original RMAIL was restricted to ITS, and the sources appear to have been lost.
The internal format of BABYL mailbox files was retained when GNU Emacs was created, even though the facility was renamed “RMail”.