Problem sending from alias to address on alias domain

I have a MIAB where I “host” domain1.tld and use that for all my primary mail via a user kim@domain1.tld.
I do own another domain (lets call it domain2.tld) which I can unfortunately not move to the MIAB server and hence I forward all mails to kim@domain1.tld.
Now I want to reply as kim@domain2.tld and added an alias for that via kim@domain2.tld -> kim@domain1.tld.
This works perfectly for all cases, except when I want to send to another user on domain2.tld. In that case I get :

SMTP Error (550): Failed to add recipient “user@domain2.tld” (5.1.1 user@domain2.tld: Recipient address rejected: User unknown in virtual mailbox table).

I can guess why this happes as it only know of my kim@domain2.tld address, but how do I get around this problem?

I suppose sending mail to kim@domain2.tld also doesn’t technically work… It never passes through domain2’s mail servers.

Fixing this is going to require changing Postfix’s configuration, which gets difficult. MiaB will overwrite your changes every time you upgrade.

First, here are the Postfix docs on turning off the local recipient checks:

That’s probably sufficient if kim@domain2.tld will only email other people at domain2.tld. If you try to send your domain2 messages from your domain1 mailserver, the DKIM/SPF records won’t match and they will rightfully be dropped for trying to impersonate another mail server.

One solution would be for domain2 to add domain1’s mail server to their SPF/DKIM records. I’ve never heard of MiaB being used like this so there may be some other issues but – in theory – it could work.

Another solution be for you to set up domain1’s mail server as a satellite for domain2, and have it forward email to domain2 delivery. That gets a little complex because you don’t want ALL your outgoing email to be forwarded to domain2, just the messages that claim to be from domain2. I’ve never heard of MiaB being used like this either.

So… how badly do you want this? : )

Pretty badly as domain2 is my primary domain mail domain and also used by the people i send most mails to.
Domain2 does not have any spf/dkim or similar setup that shouldnt give any problems.

Wow, you find you can get reliable delivery without DKIM/SPF? I find that a bunch of my messages go right into spam without them. (especially on Yahoo, but f those guys)

Sure, turn off the local recipient checks and see if it works.

Luckily Yahoo is mostly an american thing so I rarely send mails to them. Most people here in Denmark use an ISP provided smtp server for sending mail and whatever webhost they choose for hosting their domain and mail. So DKIM/SPF is not really that used around here yet for some reason. They are pushing for DNSSEC on danish domains now but that has only just started like a month or two back.
Spam is a problem, but not that big of a problem (yet).

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