MX Records Explained

MX (Mail Exchange) records are DNS records that specify which mail servers are responsible for accepting email messages on behalf of a domain. Understanding MX records is essential for understanding how temporary email services work.

What are MX records?

When someone sends an email to user@example.com, the sending server needs to know which server to deliver it to. MX records provide this information. They are part of a domain's DNS configuration and tell the internet which mail servers handle email for that domain.

MX record structure

A typical MX record looks like this:

example.com.  IN  MX  10  mail.example.com.

Components:

example.com. — The domain (with trailing dot for absolute FQDN)
IN — Internet class (standard for DNS)
MX — Mail Exchange record type
10 — Priority (lower number = higher priority)
mail.example.com. — The mail server hostname

Priority and failover

Domains can have multiple MX records with different priorities. The sending server tries the lowest priority (lowest number) first. If that server is unavailable, it tries the next priority, and so on. This provides redundancy and failover.

Example:

example.com.  IN  MX  10  mail1.example.com.
example.com.  IN  MX  20  mail2.example.com.
example.com.  IN  MX  30  mail3.example.com.

Mail is delivered to mail1.example.com first. If it's down, the sender tries mail2, then mail3.

How email delivery uses MX records

DNS lookup. The sending server looks up the MX records for the recipient domain.
Priority sorting. MX records are sorted by priority (lowest first).
Connection attempt. The sender tries to connect to the highest priority mail server.
Delivery or retry. If connection succeeds, the message is delivered. If it fails, the sender tries the next priority.
Temporary failure handling. If all servers are temporarily unavailable, the sender queues the message for retry later.

Temporary email and MX records

Temporary email services rely on MX records pointing to catch-all mailboxes. The MX records for domains like emailgenerator.pro point to mail servers configured to accept mail for any local part. This is why any random address works instantly — the MX configuration does not require pre-creating individual mailboxes.

The MX record might look like:

emailgenerator.pro.  IN  MX  10  mx.emailgenerator.pro.

All mail for @emailgenerator.pro goes to mx.emailgenerator.pro, which has a catch-all configuration that accepts any local part.

Common MX issues

Missing MX records: If a domain has no MX records, delivery fails. Some systems fall back to the A record, but this is not reliable.

Incorrect priorities: All MX records with the same priority removes the failover benefit.

Pointing to non-existent servers: MX records pointing to unreachable servers cause delivery failures.

TTL issues: Long TTLs can cause delays when MX records need to be changed.

Questions

What happens if a domain has no MX records?

Delivery typically fails. Some systems fall back to the domain's A record and attempt delivery to the web server, but this is non-standard and unreliable. Always configure MX records for domains that receive email.

How long does it take for MX record changes to propagate?

MX record changes typically propagate within minutes to hours, depending on the TTL (Time To Live) of the previous records. During propagation, some servers may use the old records while others use the new ones.