Mail relay for just one domain. zimbra 8.8.15

[zim_mike]

I'm reading a LOT of posts and articles on how to set up mail relay for one single domain but I'm nervous about trying any of what I find. Too often, articles have old or just wrong information and I cannot break this mail server.

How can I relay all outgoing mail from one domain on the server to smtp.com for example in a way that will not be lost due to an upgrade.

[Klug]

If you want this to survive upgrades (and use all postfix routing possibilities), don't do it within ZCS.

Create a new tiny VM running Postfix only (chose an OS with postfix >3, such as Ubuntu 18.04).
Setup this postfix instance the way you want (allow relay for one domain, force transport for other domains depending on recipients or senders, etc).
Then in ZCS, define the instance as outgoing relay.

This way you can modify the instance setup with no impact on ZCS and the other way round.

[zim_mike]

Thanks, that's a good idea but it means having to maintain yet another server.
If I could do this from the main mail server, it would be simpler.

I did find the following which does it from the command line but since you have to edit files, it seems it would not survive updates.

https://wavemotiondigital.com/2020/03/3 ... -provider/