MX records, explained

An MX (Mail Exchange) record is a DNS entry that names the servers responsible for receiving email for a domain, along with a priority value. Without at least one valid MX record, no mail can be delivered to that domain at all.

What an MX lookup returns

A lookup for a domain typically returns entries like:

• priority 1 — aspmx.l.google.com
• priority 5 — alt1.aspmx.l.google.com
• priority 10 — alt2.aspmx.l.google.com

Lower numbers are tried first; the rest are fallbacks. Verification uses the highest-priority host to open the smtp verification conversation.

Why MX checks come early

Checking MX before SMTP is cheap and eliminates a large share of bad addresses instantly:

• Typo domains such as *gmial.com* often have no MX record.
• Parked or expired domains resolve in DNS but accept no mail.
• Domains that stopped using email entirely fail here.

MX records also reveal the mail provider

The MX hostnames tell you who actually runs the mailbox — Google Workspace, Microsoft 365, Zoho, Proofpoint, Mimecast, or a self-hosted server. That matters because providers behave differently during verification and have different tolerance for probing. The email provider scorecard breaks down what to expect from each.

Common MX problems

• No MX record. Mail cannot be delivered. Some servers fall back to the A record, but most modern senders treat this as undeliverable.
• MX pointing to a hostname that does not resolve. Guaranteed bounce.
• MX pointing to localhost or 127.0.0.1 — a deliberate "this domain does not accept mail" signal.
• Security gateway MX (Proofpoint, Mimecast, Barracuda) — the gateway often answers 250 for everything, producing catch-all behaviour.

Check any domain's mail setup instantly with the free email verifier.