I'm a little stumped on this one. I have a mail server that was multi-homed, and used the external interface for the virtual smtp server.
I have finally installed a new firewall, and have been able to get RPC over HTTP through the firewall, as well as OWA. I created rules for SMTP and HTTPS, and forwarded the traffic to the internal interface, as well as change the Virtual SMTP server to use the internal interface.
As I mentioned, OWA and RPC over HTTPS both work; but I have NO incoming email. I am also running GFI MailEssentials and MailSecurity; but in both cases they are directed at the Virtual SMTP Server. I did verify that the incoming mail stopped as it was not even hitting the MailEssentials.
As soon as I renabled the external interface and disabled the virtual ip on the firewall as well as the rules; I started receiving incoming emails.
Sidenote: the firewall is pfSense.