Using Outlook for Android in business environments for email and scheduling
I think I found the answer for my Droid. The messages that pile up in the outbox are the ones that required a download of some attachment (photo, text, doc, etc) prior to sending. If I forced the email in my inbox to fully load prior to forwarding or replying it sent just fine. If it said that an attachment will be downloaded prior to sending it failed.
Hope that helps.