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Module 1 · Lesson 2 of 37

MX Records & DNS Routing

Part of The Complete Email Deliverability Course — Email Infrastructure Fundamentals.

When an MTA needs to deliver mail to user@example.com, it queries DNS for MX records.

How MX priority works:

  • Lower numbers = higher priority
  • 10 mail1.example.com is tried before 20 mail2.example.com
  • If no MX record exists, the MTA falls back to the domain's A record (RFC 5321)

Each MX hostname is resolved to an IP via A/AAAA records. The MTA attempts delivery to the highest priority server first, falling back to secondary servers if needed.

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