I recently changed a client from a Netopia DSL modem\router as both with another Linux router behind it to having he DSL modem as modem only and have put in a Mikrotik router to do PPPoE and NAT. I later found out that there were multiple IP addresses, so I manually assigned one of the extra ones to the PPPoE client interface.
I found out because they complained their mail stopped working and discovered that the MX record was one of the DSL IPs. We had originally setup port forwards. You can get to the servers from the outside just fine. Their DNS resolves to the public IP, but I make it a habit to specify the inbound interface as to not in advertantly catch traffic that should be Internet-bound (making SSH connections outside, connecting to external mail servers, etc.) I made another rule that specified the destination IP instead of the source interface (well, in addition to destination port). That, however, did not work. It just results in a timeout.
Ideas?