Support different relays for different domains

[arthef]

Is your feature request related to a problem? Please describe.

I have Mailu setup that is handles email for a few different domains. Each domain comes with SMTP relay service / mail forwarding from the DNS provider. It can be configured through config files on docker-compose setup.
However, with k8s system, config files are overridden or lost. It would be very useful to have an option to specific external relays for each domain in helm configuration.

Describe the solution you'd like

There seems to be external relay configuration available for the Mailu helm chart already but it only allows to specify a single relay service for entire Mailu setup.
The existing configuration could be expanded to allow setting up multiple relay services, each for a selected domain.

Describe alternatives you've considered

Another posibility would be to allow to “inject” relay_hosts and sasl_passwd to overrides folder of Mailu installation.