Echomail
I certainly sympathize with "custom interface" bulletin boards. The
technical information required to write a Fidonet compatible mailer is
contained in a dozen or so Technical Documents published by the
International Fidonet organization. "Published" in the sense of they try
to make it available for downloading, not in the sense that you can wrie to
them and get a bunch of helpful guidelines. I know System Enhancement
Associates (authors of the original ARC, SEAdog, AXE, and Kitten) keep them
online, but I don't have the number of their BBS handy.