supdns

MX Records

The core protocol for routing incoming emails to the correct mail server.

What is an MX Record?

An MX record (Mail Exchange record) specifies the mail server responsible for accepting email messages on behalf of a domain name. It is a fundamental part of the Simple Mail Transfer Protocol (SMTP).

Structure of an MX Record

example.com. 3600 IN MX 10 aspmx.l.google.com.
  • Host: The domain receiving the mail (usually the root domain).
  • TTL: Time to Live.
  • Type: MX.
  • Priority: A numerical value (lower means higher preference).
  • Value: The hostname of the mail server.

How Priority Works

You can list multiple MX records for a single domain. Each record is assigned a priority (preference) number.

  • Primary Server: Assign the lowest priority number (e.g., 0 or 10) to your main mail server.
  • Backup Servers: Assign higher numbers (e.g., 20, 30, 40) to secondary or backup mail servers.
  • Equal Priority: If multiple servers have the same priority, the sending server will pick one at random, effectively load balancing the incoming mail.

Important Restriction: Points to Hostname Only

An MX record must point to a hostname (a domain name), never directly to an IP address. That hostname must have its own A or AAAA record defined elsewhere.

Typical Setup: Google Workspace

A typical configuration for a domain using Google Workspace for email would look like this: 

Priority: 1    Value: aspmx.l.google.com.
Priority: 5    Value: alt1.aspmx.l.google.com.
Priority: 5    Value: alt2.aspmx.l.google.com.
Priority: 10   Value: alt3.aspmx.l.google.com.
Priority: 10   Value: alt4.aspmx.l.google.com.

Email Security & MX Records

While MX records handle *receiving* mail, you should also configure SPF, DKIM, and DMARC (all via TXT records) to secure the *sending* of mail and prevent spoofing.