Mail providers often give you two or more, so mail can still be delivered if one server is unavailable. Your provider tells you exactly which values to use, and they're normally set on the domain itself rather than on a subdomain like www.
A broken MX record is slow to show itself. A sending server that can't deliver keeps retrying before it gives up and reports a failure, and the SMTP specification says that give-up time generally needs to be at least four to five days. So mail can quietly pile up for most of a week before anyone hears about it.
One detail surprises people. If a domain has no MX records at all, senders fall back to the domain's own address, so a missing MX record doesn't guarantee that no mail is accepted.
Updated September 2026
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