At the office we are replacing Win7 machines with Win10 machines. On the last two that we have done a VIEWER.PM file has been created on the desktop. I assume that IER is doing this and based on the timing of when it started suspect IER v2.6.3.x versions . The VIEWER.PM file does not reappear on the desktop once it has been deleted; the one in the mailbox directory gets used.
Pegasus Mail is run from a shared installation on a server. Mailbox directories also reside on the server. WSSETUP.EXE is used to create the Pmail shortcuts and registry entries on the Win10 machines.