Did Brian's comment about the install location of PMail resolve this for you?  If you are still having issues please let us know, also let us know if its OK now.

I've used Pegasus Mail since (I think) v0.7-something.  It has always been stable and reliable.

Having just moved myself - and a number of other users - onto Windows 10, I find that hierarch.pm quite frequently gets "lost".  When PMail is closed, the hierarch.pm file is not saved and on reopening Pegasus, a new file is created without, of course, any of the (very many) filing trays that I have in the system.

I have taken to keeping a backup of the file and, for me, that will fairly simply return the hierarchy to what I had.  For several of my customers, this is not such a simple process.

Is this (as it seems) a problem with Windows 10 or is there a different reason?  It is occurring on five different computers that I know about, so it is unlikely, I think, to be a read/write problem or other mechanical "feature".

Is Pegasus Mail installed anywhere inside of a \Programs Files directory?

Have the mailbox directories been excluded from active scanning by the AV product?