I am running Exchange 2013 on Windows server 2012. The mailboxes were recently moved from a 2007 Exchange server. Messages sent by some users are sticking in the Submission queue. The Last Error on these messages reads: "A storage transient failure has occurred during content conversion". I've restarted the Microsoft exchange Transport service several times but no luck. Exchange spam filtering/antimalware protection is turned off as we have a hosted solution.
Strangely this problem only affects 3 (very important) users when sending email from their Outlook 2007 client. When they send their messages using OWA they go just fine. Does anyone have any insight into this?