An MX record, short for Mail Exchange record, is a type of DNS record that specifies which mail servers are responsible for receiving email on behalf of a domain. When you send an email to someone@example.com, your mail server looks up the MX records for example.com to find out where to deliver the message.
Every MX record has two parts: a priority number and a hostname. The priority determines the order in which servers are tried - lower numbers are tried first. Domains commonly publish several MX records so that if the primary mail server is down, delivery falls back to a secondary server instead of bouncing.
For email verification and outreach work, MX records are one of the most useful signals available. A domain with no MX records almost certainly cannot receive email, which means any address at that domain is undeliverable. Checking MX records before sending is a fast, free way to filter out dead addresses without sending a single message.
MX records also reveal which email provider a company uses. Records pointing at google.com or googlemail.com indicate Google Workspace; outlook.com or protection.outlook.com indicate Microsoft 365. This matters for deliverability, since different providers apply different spam filtering rules.