MX Record

MX (Mail Exchange) records direct email traffic to the correct mail servers, with priority values determining the order servers are tried.

How MX records work

• When sending email to user@example.com, the sender's server queries DNS for example.com's MX records.
• DNS returns a list of mail servers with priority values (lower = higher priority).
• The sender connects to the highest-priority server; if unavailable, tries the next.
• The receiving server accepts the email for local delivery.

MX record format

example.com.  IN  MX  10  mail1.example.com.
example.com.  IN  MX  20  mail2.example.com.
example.com.  IN  MX  30  backup.example.com.

Priority values

• Lower numbers = higher priority (tried first).
• Equal priorities = random distribution (load balancing).
• Higher numbers = backup servers used when primary fails.
• Typical values: 10, 20, 30 (allows room for additions).

Common configurations

• Cloud email: Points to provider (aspmx.l.google.com, mail.protection.outlook.com).
• Email gateway: Points to security vendor for filtering before delivery.
• Hybrid: Mix of cloud and on-premises servers.
• Backup: Secondary MX at higher priority for redundancy.

Security implications

• Changing MX records affects all email routing for the domain.
• Attackers may monitor MX records to identify email infrastructure.
• Misconfigured MX records can cause email delivery failures.
• SPF records must authorize MX servers to send email.