With Outlook 2003:
When I have multiple messages in Outbox, and click "Send/Receive", only one message is sent, and then a "send" error occurs.
For instance, sending through an SMTP server that requires SSL, the error 0x800ccc7d occurs, which refers to server not supporting SSL (this cannot be - sending just one message works fine).
I tried with other SMTP servers (without SSL) and still only one message is sent out of those in the Outbox. Also, I set the antivirus not to scan sent messages but no fix.