An Introduction of MX Records Priority

MX Records (Mail Exchange Records) are a type of DNS (Domain Name System) record that specify how email messages should be routed for a domain. Each MX record includes a domain name (which points to the mail server) and a priority value. MX records position to the servers to send an e-mail to, and which ones it should be sent to first, by right of way. The priority determines the order in which mail servers are used for delivering emails.

When an e-mail message is sent through the Internet, the sending mail transfer agent makes a DNS query requesting the MX record for the recipient’s domain name, which is the section of the e-mail address following the “@”. This uncertainty returns a list of host names of mail exchange servers accepting incoming mail for that domain, together with a preference number. The sending agent then attempts to launch an SMTP connection to one of these servers, opening with the one with the nominal first choice number, delivering the message to the first server with which a connection can be made. If no MX records were present, a second request is made for the A record of the domain as a substitute.

Key Elements of MX Records:

1. Mail Server Domain: This is the hostname of the mail server that will handle incoming email for the domain. It could be an IP address or a fully qualified domain name (FQDN).
2. Priority: Each MX record has a priority number (also called “preference value”), which indicates the order in which mail servers should be contacted. The lower the priority number, the higher the priority.

How MX Priority Works:

• Lower priority number = higher priority: A server with a priority of 10 will be contacted before one with a priority of 20.
• If the mail server with the highest priority is unavailable (due to downtime or other issues), the next highest priority server will be contacted.

Example of MX Records with Priority:

In this case, email will first be routed to mail1.example.com (priority 10). If that server is unavailable, the email will be routed to mail2.example.com (priority 20), and so on.

Importance of MX Records Priority:

• Redundancy: By having multiple MX records with different priorities, you ensure email delivery even if your primary mail server is down.
• Load balancing: In some configurations, MX records with the same priority can be used to distribute email load between different servers.

Detailed Breakdown of MX Record Priority:

1. Multiple Mail Servers and Failover Mechanism:
   • When you configure multiple MX records for a domain, email delivery systems will attempt to contact the mail server with the lowest priority first. If that server is unreachable (due to network issues, server downtime, etc.), the next server with a higher priority value (i.e., larger number) will be tried.
   • This process continues until a functioning mail server is found or all servers have been tried.
   • For example:
     • Priority 10 – mail1.example.com is the primary mail server.
     • Priority 20 – mail2.example.com is the backup mail server.
     • Priority 30 – backupmail.example.com is a secondary backup mail server.
   • In this scenario, if mail1.example.com fails, mail2.example.com will handle the incoming mail, ensuring continuous email delivery without any service interruption.
2. Equal Priority (Load Balancing):
   • If you have two or more mail servers with the same priority, the mail servers will receive email based on random selection or load balancing.
   • This is commonly used to distribute email load between multiple servers.
   • For instance, if two servers have a priority of 10, incoming emails will be distributed between them in a balanced way, preventing any one server from being overwhelmed.
3. TTL (Time To Live) for MX Records:
   • Like other DNS records, MX records have a TTL (Time to Live) value, which determines how long the record is cached by DNS resolvers.
   • A lower TTL means that changes to MX records (such as priority changes) will propagate faster across the internet, which can be useful during a migration or emergency failover situation.
   • A higher TTL can reduce the DNS load and improve efficiency, but it also means that changes to the MX records will take longer to be noticed.

Examples of MX Records in Action:

Let’s imagine the domain example.com has the following MX records:

1. Normal Operation:
   • Under normal conditions, all emails are directed to mail1.example.com because it has the lowest priority (10).
2. Primary Server Down:
   • If mail1.example.com becomes unavailable, emails will automatically be redirected to mail2.example.com (priority 20).
3. Backup Server Usage:
   • In the rare case where both mail1.example.com and mail2.example.com are down, emails will be routed to backupmail.example.com (priority 30).

Best Practices for MX Records Configuration:

1. Have Backup MX Records:
   • Always configure at least two MX records. The primary mail server should have the lowest priority, and at least one backup should have a higher priority to ensure reliability and redundancy.
2. Monitor and Test Mail Servers Regularly:
   • Ensure that backup servers are functioning properly, even if they are rarely used. Periodic testing ensures that your failover system works correctly.
3. Match Mail Server Capabilities with Expected Load:
   • If you’re using equal-priority MX records for load balancing, make sure each mail server is capable of handling the load effectively.
4. Consider SPF, DKIM, and DMARC:
   • While MX records control email routing, it’s important to implement other email authentication mechanisms like SPF (Sender Policy Framework), DKIM (DomainKeys Identified Mail), and DMARC (Domain-based Message Authentication, Reporting & Conformance) to prevent spam and ensure that your emails are delivered successfully.
5. Use Reliable DNS Hosting:
   • Your DNS hosting provider should be reliable with strong uptime, as your MX records are dependent on your DNS servers being accessible.

MX Records in DNS Configuration (Example):

In a DNS zone file, MX records look something like this:

example.com. 3600 IN MX 10 mail1.example.com.
example.com. 3600 IN MX 20 mail2.example.com.
example.com. 3600 IN MX 30 backupmail.example.com.

• The 3600 value represents the TTL (1 hour).
• The MX record type specifies the mail server to use.
• The 10, 20, 30 values represent the priority of each server.

Conclusion:

MX record priorities play a crucial role in email reliability and redundancy. By properly configuring MX records with appropriate priorities and testing failover servers, organizations can ensure uninterrupted email service even during server outages. Moreover, combining MX records with authentication protocols like SPF, DKIM, and DMARC enhances both email deliverability and security.