* fix: improve MX and SPF domain handling
* fix: do not use get_instance function
* fix: legacy use of tuples instead of MxRecord
* refactor: rename partner custom domain variables
* Have custom domains set up multiple dkim records to be able to rotate keys
* Apply suggestions from code review
* Some PR comments
* Keep dkim enabled if it is already
* Format
* PR updates
Co-authored-by: Adrià Casajús <adria.casajus@proton.ch>