MX Lookup
MX records with preference, addresses, reverse DNS and provider — plus CNAME/IP misconfigurations.
Enter a target
e.g. example.com — press ⏎ / ⌘⏎
What is MX Lookup?
MX Lookup lists a domain's mail servers in preference order and shows each one's addresses, reverse DNS, TTL and recognised provider.
It also catches the configuration errors that silently break inbound mail: an MX pointing at a CNAME (forbidden by RFC 2181 §10.3), an MX containing a bare IP, an unresolvable MX host, and every MX sitting inside one /24.
Publish at least two MX records for redundancy, ideally on different networks. With one MX, senders queue when it's unreachable and most give up after 24-72 hours.
Frequently Asked Questions
10 questionsHow does the preference value work?
Lower is tried first. With hosts at 10 and 20, senders connect to 10 and fall back to 20. Equal preferences are picked at random, spreading load. The number itself is meaningless — only the ordering matters.
Why can't an MX point at a CNAME?
RFC 2181 §10.3 forbids it: an MX target must be a name with A/AAAA records directly. Some MTAs deliver anyway and some refuse, so the symptom is silent, random delivery loss. Point the MX at the real name instead.
How many MX records should I publish?
At least two, ideally on different networks. With one MX, senders queue when it's unreachable and typically give up after 24-72 hours. Two MX hosts inside the same /24 give no protection against a network outage.
What is a null MX?
A single "0 ." record. Per RFC 7505 it means "this domain accepts no mail" — the correct configuration for non-mail domains, because senders get an immediate permanent error instead of queuing for days.
Does my MX host need a PTR record?
Not for inbound mail. But if the same host also sends, PTR is effectively mandatory: Gmail and Outlook widely reject mail from IPs with no reverse DNS.
I changed my MX — when does mail reach the new server?
After the old record's TTL. Until then some senders still deliver to the old host, so during a migration keep the old server accepting mail for a few days and forward it to the new system.
Do I really need a backup MX?
Senders queue and retry for 24-72 hours when the primary is unreachable, so short outages are already tolerated. A backup MX matters for long outages and for senders with strict queue policies.
Who should host the backup MX?
Preferably a different network and a different provider. A second host in the same data centre goes down with the first in a network or provider incident. Some providers sell a "backup MX" service for exactly this.
Can an MX record specify a port?
No. MX carries only preference and hostname; SMTP delivery is always port 25. The other ports (587, 465) are for submission, not delivery.
Why is the provider column sometimes blank?
Provider detection matches known MX hostname patterns. If you run your own server or use a provider that isn't in the list, the field stays empty — that's informational, not a problem.