Uninstalling/reinstalling IIS fixed the problem for me.
I was having the same issue, and after about 3 days working on it I tracked it down to a "User Rights Assignment Issue" on the Hub Transport Server.
The group "everyone" did not have the right "Access this computer from the network". This could be seen by an audit failure in the security log stating that the user "<mailbox server name>" did not have the required login type (login type 3).
Adding everyone to the "Access this computer from the network" right solved the issue.
In my case, users were trying to send email using OWA on Exchange 2007. When they clicked "Send", the sent email would go straight into the "Drafts" folder. A restart of the Microsoft Exchange Mail Submission service remedied the problem.
As per Microsoft: "This Warning event indicates that the computer that is running the Mailbox server role cannot submit messages to a computer that is running the Microsoft® Exchange Server 2007 Hub Transport server role". See MSEX2K3DB
for details on this problem.