What an MX record looks like
A domain typically publishes 2-4 MX records, each with a priority value. Lower numbers are tried first; equal priorities load-balance.
yourdomain.com. MX 10 mx1.yourdomain.com.
yourdomain.com. MX 10 mx2.yourdomain.com.
yourdomain.com. MX 20 mx-backup.yourdomain.com.How sending MTAs use MX
When SNDR delivers a message to user@example.com, our MTA queries DNS for the MX records at example.com, sorts them by priority, and attempts SMTP delivery to the lowest-priority host first.
If that host returns 4xx, we move to the next-lowest priority and retry. 5xx terminates delivery (the receiver has explicitly rejected the message). The retry policy follows the SMTP transient/permanent distinction strictly.
Common MX mistakes
Pointing MX to an A record — MX values must be hostnames, not IPs. RFC 5321 §5.1 makes this explicit; most receivers reject malformed MX.
No MX, only A — some receivers fall back to the domain's A record for delivery (RFC 5321 §5.1), but this is fragile and inconsistent. Always publish explicit MX.
MX pointing at a CNAME — RFC 2181 §10.3 forbids this. Always point MX at a name that itself resolves to A / AAAA records.
Why this matters for sending
MX configuration on YOUR domain is rare unless you receive mail. But MX checking is the first signal of recipient validity — an address whose domain has no MX cannot receive mail at all, which is one of the cheapest bounce-prevention checks. SNDR's suppression layer pre-flights MX for hard-bounce candidates.