What are MX records?
MX records (Mail Exchanger) define which servers accept email for a domain. Each entry has a priority; the lowest number is tried first. Without an MX record some senders try to deliver to the domain's A address, most give up. For a domain that should receive mail, MX records are mandatory.
What the checker verifies
- Presence: are there MX records? A missing MX on an active domain is critical.
- Resolvability: every MX host must point to an A or AAAA address. An MX pointing to a non-existent name is a common error after provider switches.
- Null MX: a single entry "0 ." explicitly states that the domain receives no mail. That is correct for pure web or parked domains.
- Priority and order: multiple servers with the same priority are used evenly.
Examples
Google Workspace:
example.com. MX 1 smtp.google.com.Microsoft 365
Microsoft 365 (the exact host is shown in the admin center):
example.com. MX 0 example-com.mail.protection.outlook.com.Learn about MX changes immediately
An MX change is rarely intentional if nobody announced it. DomainWarn detects changed or removed MX records within minutes, confirms the change against a second name server to avoid false alarms and reports it via email, Slack, Discord, Telegram or webhook.
Frequently asked questions
- My domain has no MX record but mail still arrives?
- Then senders deliver to the A address. That does not work everywhere and should not be the permanent state.
- What does an MX with priority 0 mean?
- The highest priority. The value itself is arbitrary, only the order matters.
- What is a null MX?
- The entry "0 ." per RFC 7505 explicitly states that the domain accepts no mail. Senders get an immediate error instead of days of delivery attempts.