Hi, in my test environment, I have 2 internet connections with its own Public IP (eth1 and eth4).
I use Uplink balancing and Multipath for failover and tell Internal network to prioritized to use eth1 for web. The failover test all went well.

Also I have set for MX records to send email (incoming) via eth4 first if fail then eth1.

Now my problem here is:
- if eth1 and eth4 are live, I could receive email fine. Form external, I could ping and telnet port 25 on both public IPs. Outgoing and incoming traffic are fine.
- if eth1 down eth4 live, I could received email fine. Strangely, I could ping and telnet to both public IPs.
- if eth4 down and eth1 live, I couldn't received email nor ping nor telnet on both public IPs BUT outgoing traffic are fine (works for failover).

Where could be the problem?

Thanks in advace!