Setup mail-server as a dumb relay host, ie forward any incoming mail to a primary server

[pimlie]

Is it possible to setup mail-server just as a relay host itself, similar how you can configure postfix to relay/forward incoming mail to another server using transport_maps, relay_domains and relay_recipients?

A use-case for this would be a single/primary mailserver mx10 which runs in your own office/home, with a fallback mailserver mx90 which runs on f.e. a VPS. Where mx90 should not run JMAP/IMAP, should not have access to or store any data (except in the buffer, see next) and should not be configurable itself. The only role mx90 would have is as a buffer when mx10 is for whatever reason not reachable so that emails can still be received while mx10 is down so you don't lose any emails.

It seems possible to do this already by setting up a cluster with an s3 blob storage, but somehow that feels like overkill because that requires a S3 compatible blob storage. And afaik it also means that mx90 has full access to all data in the blob store, which might be less preferred for various (privacy) reasons.

TLDR: is it possible to replicate a sqlite data store to mx90 but configure mx90 without a blob store and have it forward/relay all emails to mx10?

[mdecimus]

A blob store is always required, but there is no need to use S3 for it. You can store the blobs in the local file system or in a local RocksDB or SQLite database.
Then regarding relay and routing rules, this is covered here.

[pimlie]

Thanks for the quick response!

Do I understand the RCPT page correctly that it is possible to use an expression for relay that mimicks/combines postfix's relay_domains and relay_recipients parameters, but it's not possible to replicate mx10's (sqlite) data store and just relay anything on mx90 that is considered local for mx10?
So if I would add a new email domain, I have to add it on mx10 through the admin and then also change the relay expression on mx90, similar as I have to do now with postfix?

Apologies for asking if this should've been obvious already :)

[mdecimus]

Not sure I have understood your scenario entirely but it sounds like you also need to make routing decisions based on domains. This is covered in the routing section of the outbound queue.