Start with one failed delivery. Save the bounce code, time, sender domain and recipient domain. Keep full addresses and message content private when sharing a support ticket.
- Run the mail checker with the domain after the recipient’s @ sign. Select MX only for a focused check.
- Compare every MX target and priority with the receiving provider’s current setup screen. Lower priority numbers are tried first; equal numbers can share delivery. Old-provider records can send mail to the wrong service.
- If the result is
0 ., the domain deliberately announces that it accepts no mail. If that is unintended, replace the null MX with your provider’s records. Do not combine null MX with ordinary MX records. - Use the DNS checker to inspect A and AAAA for each MX hostname. A mail target needs usable addresses and should not be a CNAME alias. Missing MX is not conclusive: SMTP can fall back to the domain’s own address records.
- Check the provider’s mailbox, alias, quota, quarantine and service status. DNS alone cannot establish that a mailbox exists or can receive mail.
After correcting the authoritative zone, allow cached answers to expire and retry a normal message from an account you control. Avoid repeatedly querying the checker; its safety limits are shared.
References: SMTP routing and null MX.