There are many many reasons mail can 'stick' in the outbox. For one thing,
add-on's that 'touch' items in the outbox can cause mail to hang there.
Clicking on an email that is in the outbox can cause it to hang there.
ISP's sometimes change their policies without notifying you. For instance,
BellSouth in my area used to require that you use named credentials for all
outbound mail. So you had to plug your user name and password into the
advanced tab. Then they stopped requiring that, and did not tell me, and for
2 days all of my mail stayed in my outbox because their SMTP server no longer
accepted any type of authorization other than having their IP address range.
A helpful troubleshooting step is to create a second copy of the account
from scratch and see if that works. Sometimes a setting gets corrupted in a
profile. Another helpful step is to create a new account in a different
client, e.g. Outlook Express and see if that one works.
Sometimes you can drag a message from the outbox to the drafts folder and
then open it from there and resend it.
Anyway try some or all of these tips and let us know how it goes.
Bob