We have an Distribution list on Exchange 2010. It contains both internal AD accounts and some external contact email addresses. When we send to the group it hands off inside and out and everyone gets it. We had one of the external users who is a group member then attempt to reply to it from his side of the Internet (he assumed all the members would receive his reply) and he claims no one is receiving it. I don't see anything in the message tracker, and I thought the header was stripped once the delivery was successful to an outside SMTP server and Exchange had done its thing. Did I miss something, or does this guy have a problem outside my organization and needs to take it up with them?