Priority
Lower is tried first. Senders work up the list, so a second exchanger at a higher number is a fallback rather than a share of the load.
An MX record naming a host that has no address record looks perfectly fine in a table. Mail to it fails. This lookup resolves every exchanger it finds and says which ones can actually receive.
Lower is tried first. Senders work up the list, so a second exchanger at a higher number is a fallback rather than a share of the load.
A single . at priority 0, per RFC 7505. It declares that the domain accepts no mail, so senders reject immediately instead of retrying for days.
Senders fall back to the domain's A record. Poorly supported, and it breaks silently the moment the web host changes.
RFC 2181 §10.3 says an MX must name a host directly. Most servers follow the alias anyway; the ones that refuse produce failures from only part of the internet.
One is usually enough. Senders queue and retry for days, so a short outage at a single exchanger is invisible to everyone involved — mail arrives late rather than not at all. Most hosted providers publish a single name that resolves to several addresses in several places, which gives the redundancy without a second record.
A second exchanger at a higher priority number helps when the first is down for longer than senders are willing to retry, and it is worth having if you run your own mail. It is also one more thing to keep patched and configured identically; a backup exchanger that accepts mail and cannot deliver it onward is worse than none.
If a domain does not receive mail, publish a null MX. It costs nothing and it means a mistyped address bounces immediately with a clear answer, instead of sitting in a sender's queue for several days before giving up.
Usually, via the fallback to the A record, which is why the omission often goes unnoticed. It is poorly supported, it sends mail to whatever server hosts the website, and it breaks silently when the website moves. If a domain receives mail, publish MX records.
The exchanger's hostname has no A or AAAA record, so there is no address to connect to and mail cannot be delivered to it. If it is the only exchanger, the domain cannot receive mail at all. Almost always a typo or a host that was decommissioned without the MX record being updated.
Only relatively. 10 and 20 behave identically to 1 and 2. Equal numbers mean senders choose between them, which does spread load, but a mail server is not a web server and the distinction rarely matters.
Because the single name resolves to many addresses across several locations, which achieves the same thing with less to go wrong. Counting MX records is not a measure of resilience.
Getting to an enforced DMARC policy means finding every system that sends as you — the CRM, the invoicing platform, the scanner in the corner — and authorising them before you tell the world to reject the rest. We do that migration, and then the inbound side that display-name spoofing walks straight past.