We have one user that seems to be having inbound mail issues. The user has had outside users say they have sent message but they are not being delivered to the inbox. The external users did not get any message saying it was returned or blocked, we looked at spam filtering issues, even tried turning filtering off, there isnt anything sitting in the queue for delivery. Have tried reproducing the issue but it seems random with no direct cause. Any suggestions to isolate and troubleshoot this further?

I would use message tracking. There's a very good chance the message is indeed being delivered and the user doesn't see it due to a rule or folder view, has another client attached that's downloading the messages from the mailbox, a misconfigured mobile device, or something like that.Ed Crowley MVP "There are seldom good technological solutions to behavioral problems."

If it's a random issue, I'd also consider turning on protocol logging on the internet receive connector and monitor for a while. When the issue occurs check to see if the message made it to Exch. If not then it may be an issue before the message actually arrives into your ORg. If you have a gateway, then check on that too, see if the message actually get there, could be an issue with the sender or the hops along the way. If the message arrived at Exch, then use MT to track down the message and see if it got deliverd to the mbx, f you does then you can focus on the client side.Sukh