Deleting even a single message from the Main folder triggers an error message that "Pegasus has encountered a consistency problem in the folder whose details are shown above", (this being the Main folder, FOL067AB.pmm). I am able to recover consistently by re-indexing the folder, but the file I deleted reappears. What am I doing wrong? (And, be warned, I am an old man with a scant understanding of computers...)

You are not doing anything wrong. Deleted messages set in "deleted space" which grows to certain size (default is 24000 bytes) then gets automatically cleared. When a reindex occurs, that deleted space gets included in the reindex resulting in those messages reappearing.

If the consistency problem is occurring every time you delete a message then there is a problem with the folder file. You will need to check the size of the FOL067AB.PMM file using a file explorer to see if it has reached 2 GB (above 2,000,000 KB) and let us know what you find. If you post back, let us know what your level of comfort is with working in a file explorer (eg: copying a pasting files from one directory to another). Fixing the problem can involve either restoring the default Main folder or creating a new folder to serve as the Main folder. Restoring the default requires file manipulation (copy/paste two files from one directory to another), creating a new Pegasus Mail folder to serve as Main does not. It is a personal choice. Neither option is better than the other as far as function goes.

First, I am deeply grateful to you for your very solicitous assistance - the norm in help fora like this is to dismiss less technically competent folk out of hand. In actuality I am not quite as lost as I may have implied - I am entirely comfortable rooting around in file explorer.

That said, I fear we have a slightly more difficult problem here: the FOL067AB.PMM file is well short of the 2 GB limit. It is indeed large, but is actually only 1,320,095 KB at present. (For what it is worth, FOL067AB.PMI, which I suspect is the index for this particular file, is 599 KB. Not likely to be relevant, but there it is, in case.)

I have no problem archiving the existing Main folder and creating a new one if that is easier, (rather than trying to fix the existing one - the folder functions perfectly well. It is just that I cannot delete anything without throwing the index in a tizzy.) Alternatively if there is some kind of rebuild of the existing Main folder which will fix it, I am equally game to go that route - whichever you feel is easier.

And, again, thank you so much for your generous assistance.

I prefer getting things back to default so let's go that route. My instructions include renames and copies in order to preserve the original Main folder but a backup of the mail directory first would be prudent.

1. In Pegasus Mail, rename the Main folder to Main_bad.
2. Shut down Pegasus Mail.
3. Use a file explorer to copy the files main.pmm and main.pmi from C:\PMAIL\PROGRAMS\DEFAULTS to the new mail directory (same directory that the FOL067AB.PMM file).
4. Start Pegasus Mail
5. You should now see an empty Main folder in the folder list.
6. Copy all off the messages from the Main_bad folder to the new Mail folder.
7. Check the functionality of the new Mail folder (open, close, read some messages).
8. Fix any filters that were directing messages to the Main folder. You need to edit each one and redo the action, selecting the new Main folder. This is required even though the action may say Main because filter uses an internal folder ID, not the folder name.
9. You can delete the Main_bad folder once you are comfortable that everything is working normally.

I followed your most excellent instructions, (with only a few puzzlements). The process ran perfectly, with only one hiccup which I do not understand, (it choked about 2/3 of the way in copying messages, saying it didn't have sufficient disk space. There was more than ample free space on the disk, and when I went back and tried copying only the missing messages, it ran smoothly and fully). As best I can determine, the "new" Main folder is fully functional, deletes appropriately, and has no problems with indexing. I think I will keep the old file for a few more days, and then delete it.

Thank you so much.

I apologize for the puzzlements. I am glad you were able to work through them to resolve the problem.