MX records tell other mail servers where to deliver messages for a domain. The number in front of each host sets its priority, and senders try the lowest number first.
Most domains publish at least two MX hosts. When the first one is down, senders retry the next one by priority and the mail still arrives.
Each MX host must resolve to an A or AAAA record and must not be a CNAME, or some senders refuse to deliver.
Senders try the host with the lowest number first. Hosts with equal numbers share the load.
MX records only receive mail. For sending, receivers check SPF, DKIM and DMARC in TXT records and the PTR record of the sending IP.
Senders switch to the new servers once the old records expire from their resolvers' cache, within the old TTL.
No. An MX points to a host name with an A or AAAA record. With an IP or a CNAME in its place, some senders will fail to deliver.
That is a Null MX: the domain states it accepts no mail. Senders bounce the letter at once instead of retrying delivery.
