I have users encountering this issue as well and they all have one thing in common: They are sending to a collection of email addresses internal to our institution, one of whose email addresses doesn't exist anymore.
When the sender attempts to send under these conditions, the people in the to line, prior to the account that no longer exists, will receive the email, but those after do not. At that point, the email remains in the sender's outbox until deleted.
I am able to consistently reproduce this issue, both in Entourage 2004 and 2007, by writing to addresses known to not exist on our internal email system.
I have found no way to configure the client to ignore the fact that the specific accounts do not exist and move forward with sending to the others, nor do I understand why the client encounters this issue rather than having the server just take the messages and then send the sender an undeliverable notice for the address that does not exist.
Any assistance would be appreciated.
Thank you,
Branden Ohlinger