Ok, I might need some serious Expert help here......Here's the scenario. I run a company of about 100 people with Exchange 2003 behind a Sonicwall firewall. Been running just fine for 6 years plus. This past week we sub leased some space out to another company. I set them up on a completely separate network and subnet using different switches and plugging them into a separate interface on the firewall.....So basically two segregated networks that only share the same firewall and Internet connection. They installed an Exchange 2008 SBS server on their side. It's up and running fine. They currently have two MX records for their domain. The first one, with a priority of 10 points to a public IP address that I gave them to use for their Exchang/mail. The second one points to their "old" Network Solutions mail server and has a priority of 90. When we send mail to them, it ONLY goes to the Network Solutions host, not their Exchange server. I've setup a rule on the SonicWall to NAT that public IP address to their internal server, and they get mail from all other domains just fine, but not ours. If they check their network solutions email, the mail we send DOES show up there........And then they can reply to us just fine. If they try to send to us from Exchange/Outlook, it does not come through either......I'm totally stumped. I'm *thinking* it's some issue with the firewall thinking that it's a loop or something, however we never get NDRs or anything. Spam and/or Junkmail has been checked and is not the issue.......Suggestions?