Hi. we have at least now, 2 domains sending or replying to emails sent by one of our users and we dont see it come. I dont know where to start troubleshooting because i dont see their replies hitting our filter. The odd thing is that they dont get any errors/NDR either. Its like the email went out from their client and then gone..
I have verified that our domain is not listed on any popular spam databases. We also have a Barracuda Spam filter.