There are several implementations with differing legal statuses. I'll
speak only for my own.
I have a complete implementation that includes patches for 15
commonly-used UNIX mailers, including BSD Mail, MH, XMH, Elm, Gnumacs
Rmail, Andrew, and many others. Unfortunately, I cannot currently give
my implementation away. However, I was recently told that Bellcore
will probably be agreeing to put my implementation into the public
domain later this month. If and when that happens, I will announce it
far and wide, including, to be sure, on this mailing list. -- Nathaniel