See where a domain's mail is actually delivered, and whether every host in the list resolves.
MX records name the servers that accept mail for a domain, each with a preference number. Lower is tried first. A sender works down the list until something answers, which is why a second MX at a higher number is useful redundancy and why every host in the list needs to actually work.
The failures worth catching are the ones that look fine in a DNS panel. An MX pointing at an IP address rather than a hostname is invalid, and some senders reject it while others accept it, so the symptom is mail that arrives from some places and not others. A target with no address record is a hole that nothing warns about until messages bounce. A null MX, a single dot at preference zero, is a deliberate statement that the domain accepts no mail, which is correct on a sending-only domain and catastrophic on one people are emailing.
Leftover verification records are the most common untidiness. Providers ask you to add a placeholder MX to prove you own a domain, usually ending in .invalid, a reserved suffix that never resolves by design. Verification finishes, the record stays, and years later a checker reports it as a broken mail host. Sitting behind working servers it affects nothing, but it should have been removed with the verification it served.
Not strictly. Senders queue and retry for days, so a short outage on a single MX is survivable. A second at a higher preference removes that risk, and it is worth having if mail matters to the business.
Only if you genuinely want senders to pick one at random and share load. Most people who add a second MX mean it as a fallback, which needs a different, higher number.