SMTP relay and DMARC

[Stoffe]

Behaviour

Steps to reproduce this issue

I tried setting the POSTFIX_RELAYHOST expecting all emails to be sent thru the SMTP relay host. This seems to be working when sending an email to anonaddy (and it relays it to the email as it should), however, when replying to an email, it seems like its not going thru the SMTP relay, as no email is recieved in my SMTP server however I do get a reply email that my DMARC is not working. This should not be the case as the DMARC is properly set on my smtp server, however not on Anonaddys docker container. (So the email is recieved in anonaddy and then it tries to send it to the origional recipient, but does not seem to honor the SMTP RELAY option)

Why/where is it checking the dmarc server and towards what value when using this setting?

Expected behaviour

I would expect no DMARC/SPF etc check for using external SMTP server as the DMARC/SPF is not on the current machine and it does not know its proper address. (its going thru a internal relay before ending up in the proper smtp server)

Actual behaviour

Its trying to verify an unknown value against SPF (im guessing) but not the proper SMTP server value. And then bouncing the email back to the sender (me).