I included my exwrite.c at the end of this file for a start. Typically, .exwrite is used to log all incoming Bitnet messages. You can of course blow it up to send messages back to the sender when you're out to lunch, etc. BTW, .exwrite is called with the user ID of the receiving user, so it's no real security hole. Copy a file to the specified Bitnet user.
More details are in the manual page.
When is unspecified, standard input is used. If is unspecified, standard output is used. These commands can be used to query node status, lists of logged-on users, time and some other things. SMSG CMD The under CMS is the Bitnet control process. You're on your own here.