A record

What A record means

An A record is the DNS resource record type, defined in RFC 1035, that returns a 32-bit IPv4 address for a name. Its IPv6 counterpart is the AAAA record, from RFC 3596. In a zone file it reads mail.example.com. 3600 IN A 192.0.2.10, and a resolver returns the address to anyone who asks for it.

Mail depends on address records twice over. An MX record names a host rather than an address, so a sending server resolves that host's A or AAAA record before it can open a connection; RFC 5321 requires the MX target to be a hostname with an address record, which is why an MX pointing at an IP address or at a CNAME is broken even when the MX itself looks well-formed. Outbound, the address a connection arrives from is the address a receiver looks up in reverse to find a PTR record.

When mail to a domain fails at the routing stage, resolve the MX target before anything else. A name that returns NXDOMAIN, and a name that returns an address where nothing is listening on port 25, produce the same symptom as publishing no MX at all — mail queues at the sender, then bounces days later.