Conflicting nameservers.
HOW IT HAPPENS
Nameservers from two providers listed at the registrar, usually during a half-completed migration.
WHAT IT PRODUCES
Genuinely inconsistent results. Whichever nameserver a resolver happens to ask determines the answer.
Some visitors reach the old server, some the new, unpredictably, and it does not resolve with time.
WHY IT IS CONFUSING
It looks like propagation and behaves differently. Propagation settles; this does not.
HOW TO SPOT IT
WHOIS shows the nameservers. If they belong to two different providers, that is the problem.
THE FIX
Decide which provider holds DNS, ensure the zone there is complete and correct, and list only their nameservers at the registrar.
WHY IT MATTERS FOR MAIL
Mail delivery becomes unpredictable, with some messages reaching one server and some another.
Genuinely difficult to diagnose if you do not know to look.
PREVENTING IT
Replace nameservers entirely rather than adding to the existing list.
Some registrar interfaces make it easy to add rather than replace.
