Sometimes when there has been an outgoing message problem, OE will continue to not send messages that are in the outbox even when all is kosher.
Move all messages in the OE outbox to another folder.
Create a new message and try sending again.
If you cannot see the folder list, click the Inbox icon at the upper left of OE's workspace.
Still no joy:
To eliminate OE account errors, I would go to the email accounts in OE (Tools > Accounts > Mail Tab) and re-create from scratch a copy of your email account & settings. Use the known settings from the ISP web-based email login and avoid copying directly from the uncooperative OE account.
Hope this is of some use, BRL. Cheers.
Good Luck!