Hello,
you want to use proxmox for doing that? Proxmox uses Postfix, so you can configure such a scenario. But I wouldn't recommend this. I think it would be much better to setup a small container / vm with for example Debian and use this one as mail relay. In postfix you can configure transport maps where to route which domain. You can configure this in postfix on proxmox also. But I don't know what happens if proxmox become an update or new features or a change of the MTA. Better to use a software only for its solution and separate other services.
An idea to config postfix
In postfix main.conf
Code:
relay_domains = DomainA, DomainB
transport_maps = /etc/postfix/relay_maps/etc/postfix/relay_maps
Code:
/.*@domainA.*/i relay:[serverA]:587
/.*@domainB.*/i relay:[serverB]:587Better you have a look to a postfix forum for configuration I think.
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