Definition
An MX (mail exchanger) record is a DNS record that names the servers responsible for receiving email for a domain. When someone sends mail to [email protected], their server looks up the MX records for example.com and connects to one of the hosts listed.
Each MX record has a priority number and a hostname:
example.com. 3600 IN MX 10 mx1.example.com.
example.com. 3600 IN MX 20 mx2.example.com.
Lower numbers are tried first. The higher number is a backup if the first server cannot be reached. Domains hosted on Google Workspace or Microsoft 365 point their MX records at those providers' servers.
If a domain has no MX record, the SMTP standard (RFC 5321) says senders may fall back to the domain's A or AAAA record. A domain that does not want mail at all can publish a null MX to say so explicitly.
Why it matters
No working mail server means no delivery. An address at a domain with no MX record, no usable fallback, or a domain that no longer exists will bounce every time, no matter how real the name looks.
The MX record also reveals who hosts a company's email, which helps explain verification results. Some providers are easy to check at the mailbox level. Others sit behind security gateways that accept everything or refuse to answer.
How SimpleCold handles it
Domain and MX lookups are among the first checks SimpleCold runs, before any connection to a mail server. If a domain does not exist or has nowhere to deliver mail, the address comes back Undeliverable with the status no_mail_server. When a domain looks dead, SimpleCold confirms before marking it, so a brief DNS hiccup does not wipe out good addresses. Results also include mxLocation, the country where the mail server is hosted.
Look up any domain with the free MX lookup.