What does the MX priority number mean?

Lower number = preferred. A sending server tries the MX with the lowest priority first. If that fails, it tries the next one up. Same priority means round-robin (load balanced across hosts). Common setup: `10 mail.example.com`, `20 backup.example.com`. The backup only gets traffic when the primary is down.

MX returns 'No MX records found' but the domain receives email. How?

RFC 5321 says when no MX exists, sending servers fall back to the A or AAAA record of the apex domain and deliver there on port 25. Small setups skip MX and just have the apex A record receiving. Less reliable, no priority control, but still legal. Use DNS Lookup to verify the A record in that case.

The MX exchange shows `.` (single dot) with priority 0. What does that mean?

That is the Null MX record (RFC 7505). It explicitly says 'this domain does not receive mail'. Properly configured for domains that should never send or receive email (parked names, redirect-only domains). Reduces spoofing risk because the receiving side knows the domain is not supposed to send mail either.

Where does the data come from?

The MX query runs from the packet.guru server, not from your browser, so the answer bypasses your browser and OS cache. The answer still respects the record's TTL on the server side, so a recent MX change can take a few minutes to show up here.

What comes after MX lookup?

For a full mail-stack audit (SPF, DMARC, DKIM, BIMI plus a grade and recommendations) use Mail Security. To inspect individual TXT records (SPF on the apex, DMARC on `_dmarc.example.com`) use TXT Lookup.

Free, no signup, rate-limited per IP.

DNS

MX Lookup

Mail exchange records

Input

Identify the email servers (Mail Exchange) responsible for a domain. Checks priority values and hostnames.

Terminal

Console ready. Execute a command to see output...

About MX Lookup

Mail Exchange (MX)

MX records tell the world "Send email for this domain to THIS server." If these are missing or incorrect, you will not receive email.

Understanding Priority

MX records have a "Preference" or "Priority" number (e.g., 10, 20).

Lower numbers = Higher priority.
Sending servers try the lowest number first. If it fails, they try the next one.

How to use MX Lookup

Enter the domain
Apex domain (example.com). No protocol, no slash, no subdomain unless that subdomain has its own mail setup.
Hit Lookup
Records appear sorted by priority, lowest first.
Read the result
Each line is priority exchange.hostname. Multiple lines mean redundancy or failover paths. A single line 0 . is the Null MX (no mail).
Follow up
For a full audit (SPF / DMARC / DKIM / BIMI plus a grade) use Mail Security. For just the SPF or DMARC text record use TXT Lookup.

Frequently Asked Questions

: Lower number = preferred. A sending server tries the MX with the lowest priority first. If that fails, it tries the next one up. Same priority means round-robin (load balanced across hosts). Common setup: 10 mail.example.com, 20 backup.example.com. The backup only gets traffic when the primary is down.
: RFC 5321 says when no MX exists, sending servers fall back to the A or AAAA record of the apex domain and deliver there on port 25. Small setups skip MX and just have the apex A record receiving. Less reliable, no priority control, but still legal. Use DNS Lookup to verify the A record in that case.
: That is the Null MX record (RFC 7505). It explicitly says 'this domain does not receive mail'. Properly configured for domains that should never send or receive email (parked names, redirect-only domains). Reduces spoofing risk because the receiving side knows the domain is not supposed to send mail either.
: The MX query runs from the packet.guru server, not from your browser, so the answer bypasses your browser and OS cache. The answer still respects the record's TTL on the server side, so a recent MX change can take a few minutes to show up here.
: For a full mail-stack audit (SPF, DMARC, DKIM, BIMI plus a grade and recommendations) use Mail Security. To inspect individual TXT records (SPF on the apex, DMARC on _dmarc.example.com) use TXT Lookup.
: Free, no signup, rate-limited per IP.