Outlook 2010 – Emails getting “stuck” in the outbox while simultaneously sending duplicate copies of “stuck” email.

I have a POP3 email account using Windows 7 and Outlook 2010, 64 bit.  I send email updates to about 350 contacts via a distribution list.  During the past 2 or 3 weeks, I've been getting reports that my emails are being received multiple (up to 5 times) times when I've only sent it once - there is only one instance of the email in the sent file.   I've noticed that the email seems to get stuck in the outbox and apparently is sending and resending the email multiple times during the period of time that it's stuck.  I eventually have to delete the message, and still find that people have received it multiple times.  How can I fix this issue?  Someone told me it may be a full "sent" file, so I've deleted the contents of that file, also, but the problem persists.