Exchange 2010 outgoing email to certain domains

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http://serverfault.com – I'm hoping someone can point me in the right direction to a solution here. 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 the 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 cl (HowTos)