Exchange 2010: AD Recipient Cache and Locking Threads in EdgeTransport – Or How We Figured Out Something Wasn't Right With Mail Stuck in Submission Queue

A while ago we had an issue, wherein SCOM fired for message queue length for messages in Submission. When we went to take a look, the messages were stuck in Submission and weren't really going anywhere. Took a dump of EdgeTransport, paused Transport, and the queues eventually flushed. Opened Windbg against the dump and here's what…