Email & DNS
MX priority explained: how mail servers choose where to deliver
MX priority is a preference number used when a sending mail server chooses among MX hosts. Lower numbers are preferred, but priority does not create replication, mailbox synchronization, or automatic failover correctness. Every advertised MX must be able to handle mail for the domain as intended.
Read the complete MX set
Query authoritative DNS and record every MX hostname and preference. Resolve each hostname to its addresses and verify it is the service you expect.
A stale secondary MX can accept or reject mail differently from the primary and become a hidden delivery problem during outages.
Understand equal and unequal preference
Senders normally try lower-preference values first; equal-priority MX records can distribute attempts. Exact retry behavior is sender-specific, so do not assume deterministic load balancing.
A backup MX with a higher number still needs a secure path to deliver accepted messages onward.
Keep MX hostnames as hostnames
MX records point to names, not IP addresses. Those names should resolve to A/AAAA data appropriate for SMTP. Avoid CNAMEs at MX targets because standards and receiving implementations expect mail exchanger hostnames directly.
Coordinate reverse DNS and TLS identity on the actual sending/receiving servers separately.
Test from outside
Send representative external mail, verify which MX accepted it, inspect SMTP/TLS logs, and confirm final mailbox delivery. Remove old MX records only after their traffic has ceased.
- Inventory the whole MX set.
- Lower number means higher preference.
- Ensure every MX can process mail safely.
- Test real delivery, not DNS alone.