Get the exact MX set from the mailbox provider that actually hosts the recipients. Record priorities, target names, current routing and local mailbox configuration. Avoid combining old and new providers unless they explicitly support that arrangement.

1. Query the domain's authoritative MX records. Compare every priority and target with the intended mail provider's current instructions.
2. Resolve each MX target to usable A and/or AAAA addresses. An MX target must not be a CNAME alias under the DNS specification.
3. Check for malformed targets such as a complete email address, an IP in the hostname field or a duplicated domain suffix. Correct the authoritative records after saving their prior state.
4. Confirm the destination provider recognises the recipient domain and its mailboxes. On a hosting panel, check local versus remote mail routing for the actual service.
5. Send a controlled message from an external provider and inspect delivery logs or the complete non-delivery report. Use a permitted administrator diagnostic to investigate SMTP connectivity where necessary.

From your workstation, read the MX record and then inspect the hostname it returns. Substitute the actual target rather than copying mail.example.com. This verifies DNS only; a mail server still needs to accept the destination domain and receive network connections.

dig example.com MX +noall +answer
dig mail.example.com A +noall +answer
dig mail.example.com AAAA +noall +answer
dig mail.example.com CNAME +noall +answer

Read the result: The MX value should name a mail exchanger with usable address records. It is not an IP-address field and should not point to a CNAME alias. A valid web A record for example.com does not repair an absent address for mail.example.com.

Check the provider's required MX priorities and exact hostnames. An advertised but broken IPv6 address can also delay delivery. After DNS is correct, test an incoming message from an independent provider and inspect the receiving service rather than treating lookup success as proof of mail delivery.

Verify mail reaches the intended mailbox, replies work and the recipient domain remains correctly hosted. Check IPv6 connectivity if the MX target advertises an AAAA address.

A mailbox-not-found or relay-denied SMTP response indicates a different stage from DNS failure. Keep the response code and timestamp. Do not repeatedly change MX records when the destination account or mail routing is the actual problem.

• Publish a null MX for a domain that intentionally receives no email
• Check forward-confirmed reverse DNS for a mail server address
• Diagnose a website that fails only over IPv6 after an AAAA change
• Query authoritative nameservers directly when diagnosing DNS

• RFC 2181
• RFC 5321

• ISC BIND: dig and delv reference