I have this problem about 3 months ago. I have a CSSU 21.2011.38-1Tmaemo7.2 + kernel thumb1 and 51r. I think the problem is only in the Gmail account in my Nokia Messaging. In my Yahoo and Hotmail account I have no problem.
When I tried the reflash and using kernel stock , the same situation. Actually, when someone sends me an email (Gmail), I read it just fine, but if forwarding it, may never come to the sender.