Did you find an answer to this problem? I have 2 instances of SQL Server running with what appears to be the exact same configuration and one sends mail and the other does not. All my tests in the problem instance show as "unsent". No failures, just "unsent".

I've run all the sysmail sprocs and am thoroughly aware of all the information they provide. I've recreated the one mail profile. The accounts running the mail appear to have adequate priviledges. And, the mail runs from another instance with the same configuration? Have you found any clues?