Changed: a domain added to Mail no longer starts on Magpie's nameservers. Its page shows both ways to connect it as equal choices, with the nameservers its DNS uses now. Magpie copies the domain's records only after Use nameservers is chosen.
Choosing Use records shows the verification record to add. Switching from nameservers to records deletes Magpie's copy of the domain's records; nothing changes at the current DNS provider, and the confirmation now says so.
Magpie's nameservers cannot be chosen for a domain that one of those servers already answers for from records Mail does not manage.