MX records publish an SMTP server for a domain and a 'priority'. You can have multiple MX records and (theoretically[1]) you try the one with the lowest priority, and if it doesn't respond, try the next lowest, etc. Or (theoretically[1]) if you have 2 MX records with the same priority, you can load balance between them.

https://www.cloudflare.com/learning/dns/dns-records/dns-mx-r...

[1] yes...I know there's a ton of caveats here...