MailFixIt

Fastmail email configuration guide

Fastmail: MX records not receiving mail

Confirm that Fastmail is ready to accept mail for the domain, then make the public MX route match the values shown in Fastmail.

Problem description

A provider setup symptom may look like:

MX records not receiving mail after a provider DNS change.

Check the sending domain

What this usually means

Mail for your domain is going somewhere other than Fastmail, or Fastmail does not yet have the domain and recipient ready. Changing MX records alone does not create a mailbox.

Fix it

  1. In Fastmail, confirm that the custom domain is verified and the recipient address or alias exists.
  2. Copy the MX records shown for that exact domain in Fastmail’s setup screen.
  3. In the authoritative DNS zone, replace obsolete MX records only if Fastmail should receive all mail directly.
  4. If you intentionally use a security gateway, keep the gateway as the public MX and configure its next hop to Fastmail instead.
  5. Wait for DNS caches to expire, check the public MX answer, and send a new message to an existing mailbox.

Common mistakes

Do not mix Fastmail MX records with records from an old mailbox provider. A lower preference number is tried first; it is not a fallback switch for two unrelated mail services.

What MailFixIt can check

MailFixIt can expose missing, conflicting, or unexpected public MX records. It cannot activate the domain or create the recipient inside Fastmail.

Use the related guides if the MX route still points at an old provider or the domain has other DNS warnings.

Frequently asked questions

What does MailFixIt check for Fastmail?

It can show the public MX route and conflicting mail records. Only Fastmail can confirm that the custom domain and recipient mailbox are active.

Should I replace every existing DNS record with the provider values?

No. Confirm the exact record type and owner, preserve unrelated legitimate services, and change only the provider-controlled record required for the intended mail route.

Related email diagnostics

Email delivery delayed or rejected after a DNS migrationDiagnose email that stopped arriving after a DNS change. Compare public MX records with the intended provider and avoid false alarms for mail gateways. SPF is not configured for the envelope sender domainDiagnose SPF missing on a bounce or MAIL FROM subdomain. Check the exact envelope domain, add only authorized senders, and verify safely. DKIM public key record is invalid or revokedDiagnose an invalid or revoked DKIM public key record. Check the exact DKIM selector in public DNS before replacing a sender-provided key.

Primary sources: Fastmail official setup guidance.