redundant mail server

[acumen]

can I setup 2 machine running ISPConfig 3.x and both of them can redundant each other? e.g. server A got domain abc.com (IP 1.2.3.4)and server B got 123.com (IP 5.6.7.8) and running as mail server. If server A is down, all abc.com will forward to 5.6.7.8 and temp store in server B, when Server A up again, the mail will send back to Server A. Same when server B is down. I know this also need DNS MX record to help. I already setup MX 10 point to 1.2.3.4 and MX 20 point to 5.6.7.8. the remaining should be how can postfix receive the emails if there's no such domain record? Is there any way I can set in ISPConfig? or I need to amend relay domain in main.cf ?

[Mark_NL]

You need to tell postfix on server B that it should accept mail for abc.com, and relay it to server A when possible.

This can be done with the "relay_domains" settings of postfix.

in your main.cf put:
relay_domains = $mydestination, abc.com

Now every mail for abc.com will be accepted, server B will automatically try to relay the mail to server A, if it fails it'll try it again after some time.

[acumen]

I saw relay_domains in main.cf point to mysql-virtual_relaydomains.cf, and in that file, it point to a mail_transport in dbispconfig. After all checkings, I know that it's about Email Routing in ispconfig. So, is that means I only need to add a abc.com in Email Routing section?

In Email Routing, there is a destination field, what I have to put in? the 123.com's IP?

[Mark_NL]

Ah yes, ispc3, i forgot

you can add an Email routing for abc.com yes

then test it something like this:
- add it
- turn off postfix on server a
- "tail -f /var/log/mail.log" on server b
- send mail to someone@abc.com
- check the log if it's accepted
- turn on postfix on server a
- check logs if it relays
- check mailbox on server a