As others suggested there are some issues with split-horizon and routing protocols. Again, they can be worked around but they are not fun. That is one of the two major reasons that I use point-to-point subinterfaces except in cases where it's not p...
Yes something of the following form:ip access-list standard anythingpermit anyroute-map [name] permit 10 match ip anything set interface Ethernet0 or set ip next-hop [address of router/fw/etc on ethernet0 network]interface [serial something] ...
Where is your DNS server? I'm assuming it is the same box oas the mail server. I would start by looking at the dynamic table (show ip nat trans) when dns stops working. Since mail uses so many dns lookups, I would think you might be filling the ta...