I send emails with attachments over 2 MB and, while the email appears to just sit in the Outbox, it's actually being received by the recipient(s) multiple times. I have no indication this is happening as the email never moves over to my Sent Items. I try to delete it or move it and can't...at least not at first. After some time (often 30 minutes to an hour), it will allow me to delete or move it. Since the recipients are getting the email, I know the issue is not related to my outgoing email bandwidth through my ISP.

Is it Outlook or Outlook Express you're using? If it's the latter then it may be a corrupted Outbox. You can fix that by going to the store folder (its location is given under Tools/maintenance), rename outbox.dbx to outbox.old, restart OE & it will be recreated. The same option doesn't exist in Outlook, so my only suggestion there is to insert the 'Office' disk & do a repair. "I've always been mad, I know I've been mad, like the most of us..."