OK. I got it fixed. The message that could not be deleted was set to read only. Changed that and I was able to move the message. Then I followed your instructions on how to proceed once I had emptied the Main folder, and that all worked. So I am all set. Thanks again for all your help.

I am getting an error indicating that my main folder is too large and I need to delete some messages to recover space. I would really rather not permanently delete messages. Is it possible to export messages to a file or folder before deleting, and then to restore deleted messages from what was exported if desired? That way I could always recover any old deleted messages I may want to examine again.

Instead of exporting the messages why not store in a mail folder?

You can create a new 'Filing tray' and new folders to sort/move the messages from your main folder. Once moved to another folder the error will disappear.

 currently I have over 129 folders with a total of 415K messages (no folder with over 15k messages just to be safe) going back to 1997. Anything over 10 years old is in a subfolder of the year i.e. 2000\Bank, \2001\orders, \2001\Radio etc

If exported the messages can not be searched since they would exist outside of Pegasus.

Read the manual/help for filing trays for more details. 

I created a folder and moved a large number of messages into it, but I'm still getting the error message. Error message says file main.PMM is too large.

The folder has hit the 2GB size limit.  Additional space is needed to perform moves and that space isn't available.  Here is how to recover from the 2GB size folder limit:

1.  Move all messages you want to save to a temporary folder  and delete the remaining messages out of the affected folder.  If you want to keep them all move to multiple folders.
2.  Delete the original folder. 
3.  Rename the temporary folder to the name of the original folder  (optional).
4.  Reset any filters that pointed to the original folder.  This is must be done even if you renamed the temporary folder back to the original folder name. 

NOTE: When the Main folder is the problem folder you must reset the default mail folder in Tools > Options > General settings > Basic settings.  Use the "Select folder" button to select the new default mail folder.

Tried to follow your suggestion, but a problem came up. I moved about half the main folder messages to another folder. That worked OK. I then tried to move the remainder of the main folder messages to yet another folder, but then I got the (obviously wrong} message that there was not enough room on the drive to complete the move (there are 1.5 TB of space). I tried the move again, but this time a warning sound was given and no move was attempted. So I created a 3rd new folder and tried to move the messages to that folder, but again a warning sound happened and no move was attempted. What's going on?

You're describing the OS thinking the drive is out of space.  To confirm that I would attempt to copy the Pegasus Mail Main folder file using a file explorer and see if that works.  I suggest having a current backup of your home mailbox directory before starting.

Close Pegasus Mail

Use a file explorer to navigate to the home mailbox directory (default is c:\pmail\mail\admin).

Locate the MAIN.PMM file (Post back if you don't have one, it my be a different filename).  Right click > Copy

Paste it into the home mailbox directory.  If you encounter an inadequate space error you know its the OS and not Pegasus Mail.  If it copies, delete the copy and we'll try a different approach.  Don't start Pegasus Mail with the copy in place.

I was able to copy the file, but I am not sure I copied to where you wanted me to copy it. What is the "home mailbox directory"? In any event, I copied it into the "admin" folder, so it appeared as usual with the "-copy" added to the filename since it was in the same folder as the original file.

I decided to restore my backup of the main.PMM file and try to move the messages to other folders again, and this time it worked, except one message remains in the main. I cannot move or delete it. What should I do to be able to get rid of that one remaining message?