MX lookup
Resolves MX records, orders them by preference, and detects null MX (RFC 7505) and MX targets that do not resolve.
Updated 2026-10-07
What this tool checks
The MX lookup resolves the MX records of a domain and orders them by preference.
- It detects a null MX (RFC 7505), and a null MX combined with other records.
- It resolves the mail hosts to their A and AAAA addresses: up to 10 hosts are resolved, and up to 50 are listed.
- It flags MX targets that are IP addresses and targets that do not resolve.
- If the domain has no MX record, it says so; senders will then use the domain's address records.
How to read the result
The list is in the order a sender would try the servers. The addresses are what the host resolves to when you run the check.
Limitations
- The check does not connect to the mail servers. It cannot tell you whether a server is running or accepts mail.
- Hosts beyond the first 10 are listed without addresses.
- It does not identify the mail provider behind the hosts.
Use it from code
curl -i "https://api.probekit.dev/v1/mail/mx?domain=example.com"A request without payment is answered with 402 Payment Required and the exact price. See the API reference.
Limits
| Input | Domain |
|---|---|
| Deadline | 10 seconds |
| Freshness | Up to 5 minutes, never longer than the record TTL |
| Price | $0.01 per check |
Findings this tool can report
MX (5)
- Warning
MX_NOT_FOUNDThe domain has no MX records. - Info
MX_NULLThe domain publishes a null MX and does not accept mail. - Error
MX_NULL_WITH_OTHERSA null MX is published together with other MX records. - Error
MX_TARGET_IS_IPAn MX record points to an IP address instead of a host name. - Error
MX_TARGET_UNRESOLVABLEAn MX host has no A or AAAA records.
Check (1)
- Warning
CHECK_QUERY_BUDGET_EXCEEDEDThe check stopped after reaching its DNS query limit.
DNS (3)
- Error
DNS_DOMAIN_NOT_FOUNDThe DNS resolver reported that the domain does not exist. - Warning
DNS_RESOLVER_FAILUREThe DNS resolver returned a server failure. - Warning
DNS_RESOLVER_TIMEOUTThe DNS resolver did not answer in time.
Frequently asked questions
What is a null MX?
A null MX (RFC 7505) is a single MX record with preference 0 and the target .. It tells senders that the domain does not accept mail, so they do not wait and retry.
What happens if a domain has no MX record?
Senders then fall back to the domain's address records, an implicit MX. The check reports this case, so you can see that no MX is published.
Why is an MX pointing at an IP address flagged?
An MX record must name a host, not an IP address. Some senders will not deliver to such a record.
What do the preference numbers mean?
Lower numbers are tried first. Hosts with the same number are equal alternatives.