Problems on Windows 10 - using Pmail 4.70

I have been using Pmail for about 20+ years, first on DOS, then on Windows since the late 1990s, and it has been reliable and efficient - until I installed Windows 10.

 I have three PCs at home all in a Homegroup and with full access to all users. All users are Administrators too.

Pmail is now having what appear to be file permission problems. In particular "The user you are attempting to 'become' (aidan) does not exist on this system."

The users are all in pmail.usr and show correctly in the Users menu.

 Any ideas?

 Cheers

 Aidan