You could also refer to this thread: http://community.pmail.com/forums/post/51998.aspx where the problem has already been discussed.

Hi There,

The error below comes up every time I send an email, whether or not it has an attachment, and even though it says the message has not been processed, the email does send, however sits at the bottom of the pegasus screen as if it hasn't been sent.

ATTACHMENT ERROR:   "The file "WPM$6787.PM$' which has been attached to a message in your outgoing mail queue, cannot be located. Please edit and re-attach the file, or remove it prior to sending. The message has not been processed."

 The error occurred after I sent an email and deleted the attachments prior to sending.  I cannot locate the file "WPM$6787.PM$' anywhere......please help!!

Many thanks. 

Don't delete message and attachments until you see the stats line says "message sent"

If you queue messages before sending perhaps the message causing the error is one that is stuck in the queue.  Open the queue manager to see if any messages are listed.  To open the queue manager go to Files | Review queued mail.