I have a client, when he’s at the office, his mail server resolves to the WAN IP of the server, but it needs to resolve to the local LAN IP of the server (so he can access the shared drives, email, etc.). When he’s at home, the mail server then NEEDS to resolve to the WAP IP of the server so he can access the VPN, and shared drives, email, etc.
For some reason, it’s been a while so I don’t remember all the symptoms, but when he’s at the office, he can’t access email or the shared drives via the WAN IP. The shared drives are obvious why he can’t, but he should be able to access email. Either way, he currently changes this manually using the hosts file. I believe it’s an issue with the DNS server at the office (I believe his server also acts as the DNS server).