I just upgraded to Win 7 and I really wrestled with the WinExplorer search box before giving up.  I opted to install and use the Explorer Plugin for NotePad++;  it works very well. 

I found one .PMX message in my Home directory with a key word that  appears in the body of the message.  The message type I have is Final form.  It is a text-only message; no RTF format nor attachments nor images.  There was no corresponding .PNX file.  I deleted the .PMX file and now have an empty outgoing mail box.

 Thanks very much.

My hard disk crashed; I installed Pegasus 4.63 in Win7.  I set the Home directory up to point to my old directory.  I have one old persistent non-empty email queued.  It deletes but is still there when I review the queue again.  I have:

1. Use Help | About Pegasus Mail | Info to locate the HOME
  mail directory.

2. Use File Manager or Explorer and go to this directory and
  delete the zero length PMO, PMX and PMW files.

I have no  zero length PMO, PMX and PMW files.  All my PMO's are 1kb.  I have one 2kb PMX and no PMW's.

How can I delete my persistent email in my queue?

Look for the 'odd' .PMO or .PMX or
.PMW file; not paired with a same
named .PNX file.

Viewing the contents of those files
in a text editor may give you a clue.