For O.E. to send out email, the Sent Box must be loaded into ram memory.
If you never clean out that box, like one customer of mine, it can get so large as to overrun the free ram. No mail will be sent and you may even get an out-of-memory error.
In the case above, I deleted the Sent Items and Outbox folders along with the Deleted Items folder and everything worked OK from there on.
Those folders should be kept clean on a regular maintenance schedule.
Important email should be saved as files in "My Documents" where they will cause no problems and be safe from problems.
Good Luck,
The Doctor
General