I've added a little feature to messages. It's documented in HELP MESSAGING.
You can now set the equivalent of a voicemail message such that when people send you messages, they'll see the "returnmessage" that you have set.

The syntax is: SET RETURNMESSAGE <blahblahblahblah>.

Penned by my hand on the 25th of Scarlatan, in the year 270 AF.