What you saw was Pegasus Mail attempting to recover deleted space in that folder.  Apparently something went wrong during the process.  You should check the folder consistency and perhaps reindex it.   

I recommend reading the help file on this topic.  Find it by going the Help > Help index then under the Index tab locate the topic called "Reindex folder" 

When moving an item of mail from the Junk Mail folder which I have to do frequently because I can't get the filers to work I had a brief message to say I should leave Pegasus mail as soon as possible because an attempt to restore space had gone wrong - those may not be the right words. When I re-started Pegasus mail the folder and it's contents had vanished. I'm mystified.

I am using version 4.63 build 325.

Can anyone help?

The folder has just re-appeared with 1 item in it - what happened to the rest?