MX record

Also called: Mail exchanger record, MX.

MX (mail exchanger) records live in a domain's DNS. Each one names a mail server and a preference number; senders try the lowest number first. When anyone sends mail to office@smithplumbing.com, their server looks up the MX records for smithplumbing.com to know where to deliver it.

RFC 5321 section 5.1 covers the edge cases. If a domain has no MX records but does have an address (A or AAAA) record, the domain itself is treated as the mail server, the so-called implicit MX. If a domain publishes a "null MX" (MX 0 ., defined in RFC 7505), it is explicitly saying it accepts no mail.

MX lookups are the first real test in email verification. A domain with no way to receive mail can't have a valid address, so it fails fast and cheaply. The records also show who hosts the mail: records ending in google.com point to Google Workspace, and mail.protection.outlook.com points to Microsoft 365. That helps predict how a domain will behave in a mailbox check.

For local business lists, a missing MX record is often the first sign a business has closed or let its domain lapse. Verifiers that ignore the implicit-MX rule can wrongly mark working addresses invalid, so check how your tool handles it.

Example

Looking up smithplumbing.com returns 10 aspmx.l.google.com and 20 alt1.aspmx.l.google.com. The business uses Google Workspace, and senders try the priority-10 server first.

In Leadraft

Every email is MX-checked before the mailbox check, and pattern-guessed addresses are only suggested when the domain has a working mail server.