Puzzle! smtp return traffic to internet gets dropped due to no adjacent traffic found (guess NAT).
We have two distinct internal smart host appliance providing SMTP services (smtp1 & smtp2).
We also have two (2) distinct ISP providers behind Cisco ASA 5510 (isp1 & isp2)
smtp1 is successfully exchanging emails via ISP1, as this also correspond to default route on ASA: route outside 0.0.0.0 0.0.0.0 18.104.22.168
but smtp2 failed while trying to exchange smtp traffic.
Mainly, the return smtp traffic from smtp1 back to isp2 is actually being divert by the default route at the ASA.
We understand Cisco ASA 5510 do not support PBR (Policy Based Routing), which would have allow to detect the return smtp traffic coming from source smtp2 (192.168.1.5) and force it to get out via E0/2 using static route.
Is there a way to overcome this problem/limitation?
The idea is to be able to offer SMTP redundancy by dedicating each MX pointer to one dedicated set of SMTP host and ISP provider.
Some sites mention using two distinct ASA and one (1) router for PBR.