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MX

TL;DR: MX records tell senders which mail server receives email for a domain. relay points your MX record to the relay server and dispatches incoming mail to your webhooks.

What is MX?

MX (Mail Exchange) is a DNS record type that specifies the mail server responsible for receiving email for a domain. Each MX record has a priority value and a mail server hostname. When someone sends an email to a domain, the sending mail server looks up the MX records to find where to deliver the message.

MX records are part of the DNS specification in RFC 1035 and are used by SMTP as defined in RFC 5321.

Why MX matters

Without an MX record, a sending mail server does not know where to deliver email for your domain. Some servers fall back to the A record of the domain, but this is not reliable. The MX record is the standard way to route email.

The MX record also enables load balancing and redundancy. You can list multiple MX records with different priorities. The sending server tries the highest-priority server first. If that server is unavailable, it tries the next one.

How MX works

The MX lookup and delivery process follows these steps:

  1. The sending mail server needs to deliver a message to user@example.com.
  2. The server queries DNS for the MX records of example.com.
  3. DNS returns a list of MX records, each with a priority and a hostname.
  4. The sending server sorts the records by priority (lowest number first).
  5. The server connects to the first MX host on SMTP port 25.
  6. If the connection fails, the server tries the next MX host in the sorted list.
  7. If all MX hosts fail, the server queues the message for later retry.1

Priority values

The priority value is a 16-bit integer (0 to 65535). Lower numbers mean higher priority. For example:

example.com.  MX  10  mail1.example.com.
example.com.  MX  20  mail2.example.com.

In this example, mail1 has priority 10 and mail2 has priority 20. Senders try mail1 first. If mail1 is unavailable, they try mail2.

When two MX records have the same priority, senders pick one at random.2 This distributes load across the two servers.

The fallback to A records

If a domain has no MX records, some sending servers try to deliver to the A record of the domain itself. This behavior is defined in RFC 5321 Section 5.1.3 However, this fallback is not universal. Many modern mail servers do not fall back to A records. You should always publish an explicit MX record.

How relay uses MX

relay uses MX records in two ways:

Incoming mail

For incoming mail, relay points the MX record of your receiving domain to your sender subdomain on the relay nameserver. For example, if your receiving domain is app.example.com and your sender subdomain is mail.relay.example.com, the MX record is:

app.example.com.  MX  10  mail.relay.example.com.

The relay MX server receives the mail on port 25 and dispatches it to your configured webhooks. Webhooks follow the Standard Webhooks specification.

Outgoing mail

For outgoing mail, relay uses the SMTP server on port 587 for message submission. The MX record is not involved in outgoing mail. MX records are only for incoming delivery.

Further reading

  1. The retry schedule is implementation-specific. RFC 5321 recommends at least 4 to 5 days of retries. The sending server typically waits longer between each retry attempt (for example, 15 minutes, 1 hour, 4 hours, 8 hours). 

  2. The random selection for equal-priority MX records is defined in RFC 5321 Section 5.1. The term “equal preference” means the sender can try any of the servers at that priority level in any order. 

  3. The A record fallback is a legacy behavior from the original SMTP specification (RFC 821, 1982). Modern mail servers may still implement it, but it is unreliable. A domain without an MX record is often misconfigured or abandoned.