Clarification on relayhosts

[Syndace]

The documentation here tells me to edit a domain and to assign a relayhost to it. When editing a domain though, all I have is a checkbox to enable relaying of the domain, I can't "assign" a relayhost. I later found this issue, which clarifies that checking the checkbox makes Mailcow relay mails to the primary mail service it finds by looking at the MX DNS-records of the domain.

Please clarify that

1. checking the checkbox really does all the magic and you don't have to mess e.g. with transport maps (which is what I did first)
2. Mailcow selects the destination by selecting the primary MX entry from DNS

(Note: I might create a PR for that later, but I want to get my mail setup done first)