I have a website running on another server, and now I'd like to set up a secondary mailserver on my raq3 for that website.

I thought off fumbling with a catch-all email account, but there must be a better way to do this, I guess

Anyone knows?

Add a second MX record for that domain pointing to whatever server you want to use for the mail, with a higher priority than the primary mailserver MX record.

Wait, this might not be what you want... can you explain exactly what you are trying to do, need?

The goal is pretty simple:
Add a second mailserver in case the first one is offline.

And what I want is for the secondary server to automatically forward all the mail it received to the primary server

With a catch-all account on the secondary I can set up the primary server to collect the mail, but I want it the other way round :)