So we upgraded from 10.5.8 Mail server to 10.6.2 and everything went very smoothly. Mailstores were migrated and the transition from Cyrus to Dovecot was great. However once all the email was moved over and all the settings were double, and triple checked. People started complaining that every once in a while their email was not getting through the server, it was getting bounced back to the sender. At first I thought that it was an internal issue with the users on our network, then I got a complaint from someone from outside the organization trying to send an email to one of our users.

Great, another strange Apple bug I thought until someone was able to forward me the bounced email to my personal email account. Further insight in the error showed that the message was getting bounced back due to a blank subject line. Read more..