I installed Pmail only to read old pmail messages. When I use a program using the mapi interface, Pmail is invoked. How can I disable Pmail mapi?

Setting the default email app in your system to whatever program you want MAPI associated with should do it. Reboot after doing this.

You can then look at the Pegasus Mail entry in the "Set default by apps" list (if it is still there) to insure Pegasus Mail WSENDTO utility is not associated with any protocols.