What is / Full version
MX records
Mail exchangers are published as MX RRs. Preference orders delivery attempts. Without MX, RFC 5321 allows delivery to the domain's address records.
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Preference and failover
Multiple MX records at the same owner name may point to different hosts with different preference values. RFC 5321 expects clients to try higher-priority hosts first, but operational practice often load-balances among equal preferences.
An MX value is a domain name, not an IP address. If that name does not resolve, delivery cannot proceed even when a backup MX exists with a higher preference number.
Null MX
RFC 7505 defines a null MX that signals a domain does not accept mail. That is different from a domain that simply lacks MX records; absent records trigger the fallback to address lookups in RFC 5321.
Delivery algorithm
SMTP clients collect all MX RRs for the recipient domain, sort by ascending preference, and try hosts in that order. Equal preferences may be treated as a load-sharing set by the client implementation.
When no MX exists, RFC 5321 permits treating the domain name itself as the mail exchanger if address records exist. That fallback is not the same as publishing an MX.
Backup MX with a higher preference number still has to accept mail on port 25 for the role to work. A hostname in MX that does not resolve stops delivery for that preference level.