Sprint Treo 650 and Exchange ActiveSync

Ok, I've had a Certificate associated with my email address for quite some time. I typically send customers emails via this account (go figure), and until recently, only one or two would have an occasional problem receiving them. Of course, as I turn increase my customers' skills, they start using the devices I use. Now that some of my customers are using Treo 650s (I just moved to Treo 700w), they are having issues syncing when I send messages from my account. This only seems to happen when I include the secure certificate. If they delete that message from their inbox, syncing seems to work fine. The wierdest part is that it only seems to be sprint, and only the Treo 650...