Is this still hanging?  Try this:

1.  Locate your new mail folder (Help/About/Info) then close Pegasus Mail.

2.  Use Explorer to find the oversize message and move or delete it.

3.  Restart Pegasus Mail.

I had winpm-23 hang sending a large word doc. I found out later that it was about 250 megs.

now pm will accept id, warn me about other users, count as it opens folders, and then hangs, 'opening new mail folder',
and does io reads forever as shown in the task manager

 Is the problem in a new mail folder, or in a sending queue? How can I get pmail to unhang?

windows vista home premium, 4 gigs of ram, quad 2.6 ghz processor.