yahMAIL is one of the few WASD scripts not to work "out-of-the-box".
Check the following information and package documentation carefully.

Access  README_WASD.HTML  for WASD specific information,
        README_OSU.HTML   for OSU (DECthreads) information,
        README_CGI.HTML   for other CGI (e.g. Purveyor) servers.
