I have an Exchange 2010 server that hosts a couple of our business domains. Incoming mail is fine and 99% of the time outgoing mail is fine. I have had a couple of users claim that mail they are sending to only one or two domains is 'disappearing'. They send emails or reply to ones they receive, it goes into their outbox, and then goes to sent items as per normal. There is nothing stuck in any queues on the Exchange box... but the recipient doesn't receive anything from them.
I can't see any issues on our end and they claim they have no issues on their end, it's not caught in their spam filters etc. SO far I have two domains doing this - but everything else works perfectly, incoming and outgoing.
Just yesterday one of our users was emailing a client, replying to received emails and sending new ones... and halfway through the day the external client stopped receiving emails from our guy, even though Outlook said it was sent.
I am getting no bouncebacks, delays or any other kind of error messages in Outlook or on Exchange.