We've been trying to figure this one out for a couple weeks now. Using XF 1.5.1 and running PostFix 2.11.0. Many emails are being sent, but some users (most notably another admin) are only receiving notifications for a few select threads and conversations. I had our hosting company take a look at this and they tweaked postfix to improve it's performance some, but there doesn't seem to be a problem on their end. The server reports 0 messages in the queue. Any ideas?