MX priority, without folklore

MX priority is a preference number on your mail exchanger records. The lower number is tried first, so 10 is preferred over 20. A higher number only becomes useful when the lower one fails, and only if that host can truly queue mail. Priority is not speed: MX 0 is not faster than MX 10, and it is only more preferred.

What a real backup MX must do

A secondary MX has to accept mail for your domain when the primary is unreachable. It should queue that mail and deliver it to the primary once the primary is back. A host that immediately returns a 550, or that drops mail into a different mailbox store, is not a backup. Two MX names that point at the same pair of IPs with 10 and 20 are a duplicate, not redundancy. One MX is enough when you run a single mail host. Google Workspace, Microsoft 365, and similar services publish several hostnames; copy those names and the numbers they chose.

dig example.com MX +short

You should see one or more lines of priority hostname. If you still list MX 0 on a server you retired and MX 10 on the live one, some clients will keep trying the dead name and stall. Remove the old name. If the domain should not receive mail at all, use a null MX per RFC 7505 instead of pointing MX at a fake A record that blackholes other people’s mail.

Preference is not latency or a health check

SMTP clients try the best preference first, then worse ones after failure. They do not magically fail over with no queue on the secondary. If priority 20 answers and stores mail in a different database, users see split inboxes. If priority 20 bounces right away, senders get a bounce even though priority 10 would have worked on a later retry. Decommission old MX names when you leave a provider. One provider with its published numbers is the usual setup. Running two providers side by side is a real project, not a checkbox.

SPF still has to match every sender

TXT records for SPF must name every host that might send, including a backup that could bounce or relay. A long TXT record is a separate failure mode. See TXT records that are too long if SPF flattening is the next step you are considering.