Though this is not exactly a mailing scenario but since have a zimbra involved and not sure how zimbra will react curious to know if the below scenario can be possible for me?
I have say ZCS installed for example.com domain for which mail server is configured on Private IP address 192.168.10.10 and I have two ISPs configured on firewall say ISP-A and ISP-B
IP address from ISP-A is 18.104.22.168 and ISP-B is 22.214.171.124 and both are natted to same Internal IP address on my firewall having two MX records added on INTERNET i.e.
126.96.36.199 mail.example.com 10
188.8.131.52 mail1.example.com 20
Now if one of my ISP goes down mails coming from foreign domain will try for first entry if that is not live then it will try for second IP which should get accepted and mails should get delivered??