Post Office and Win 98

I am trying to create a Post Office using Win 98. I have installed the messaging from the old win95 directory on the 98 CD. I can create the Post Office and add users successfully. I can even configure clients to access the Post Office and see the global directory. I can’t send mail though. When I send a message I get it returned undeliverable because file is in use. It doesn’t tell me what file is in use though. Microsoft offers little to no tech support on Microsoft Mail in a Win98 environment.I am used to working with Exchange. This is a 4 user peer-to-peer.

Any ideas??