The basics
What MX records tell you
MX records are how the internet knows where to deliver email for a domain. When someone emails you@yourcompany.com, their mail server looks up the MX records for yourcompany.com and connects to the server with the lowest priority number.
The host names also show who runs the mailbox: aspmx.l.google.com means Google Workspace, and yourcompany-com.mail.protection.outlook.com means Microsoft 365. Security gateways such as Mimecast or Proofpoint appear here too when they filter mail before it reaches the mailbox.
Common problems
MX record mistakes to look for
- Old MX records left behind after moving to a new provider, so some mail goes to a server nobody reads.
- An MX record pointing at an IP address or a CNAME. MX records must point to a host name with its own A or AAAA record.
- A security gateway in MX, but the mailbox still accepting mail directly, which lets attackers skip the filtering.
- MX set up but no SPF or DMARC, so anyone can send email pretending to be the domain.
Questions
MX lookup FAQ
What is an MX record?
A mail exchanger (MX) record is a DNS record that tells other mail servers where to deliver email for a domain. Each record has a host name and a priority number.
What does MX priority mean?
Senders try the server with the lowest priority number first. Higher numbers are backups, used only if the preferred server can't be reached. Servers with the same number share the load.
Why does a domain have no MX records?
Either it doesn't receive email, or its DNS is incomplete. Domains that never receive email can publish a null MX record (priority 0, host “.”) so senders give up straight away.
How long do MX record changes take?
Usually minutes to a few hours, depending on the record's TTL (time to live). Old values can be cached until the previous TTL runs out.