Backup Mailserver

[Tripple]

Whenever my Zimbra mailserver goes down I have a backup mailserver in a datacenter where I entered a line in Postifx main.cf:
relay_domains = $mydestination, myzimbra.com

All mail will stay there untill the Zimbra server is back online.

Can I fix this the other way around? Can Zimbra act as backup mailserver?
My guess is this will work but I have to add this line after every upgrade.

[phoenix]

Can I fix this the other way around? Can Zimbra act as backup mailserver?

Yes, it can as Zimbra uses postfix as the MTA.

My guess is this will work but I have to add this line after every upgrade.

Yes, that's correct.