A relay's settings get its MX preference, beside its public name: where
this server publishes a domain's DNS, the relays are its MX set (relay
spec, RL-25). A server from before that sends none, and the field reads
10, the server's default.
The help for Linked said turning a relay off closes its port 25. It
doesn't: the relay keeps its mail and goes on taking mail in, held, and
the help now says so. The public name's help says it goes in the MX and
SPF records.