Email troubleshooting guides
Fastmail setup guides, organized by problem
Browse Fastmail MX and SPF setup guides for custom domains.
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Fastmail: MX records not receiving mailFix a Fastmail custom domain that is not receiving mail by checking its active mailboxes and public MX route.
Fastmail: SPF record for provider sendingAuthorize Fastmail to send for your custom domain without creating duplicate or incomplete SPF records.
Primary sources: Primary technical reference.