Fastmail Email Verification

What the MX records tell you

Fastmail hosted domains publish MX records pointing at Messaging Engine infrastructure, typically in1-smtp.messagingengine.com and in2-smtp.messagingengine.com as a redundant pair. Custom domains hosted at Fastmail publish the same records, which means a business domain with no obvious Fastmail branding is still recognisable as Fastmail from DNS alone. Mailthentic uses the messagingengine.com MX signature to identify the provider, then applies Fastmail specific handling to the probe.

How Fastmail answers an SMTP probe

Verification connects to the Messaging Engine host, issues EHLO, sets a null sender, and issues RCPT TO for the address being checked. The session ends there. Mailthentic never sends DATA, so no message is generated and nothing arrives in the recipient's mailbox.

Unlike a security gateway, the host answering here is part of the platform that actually owns the mailbox directory. That is the crucial difference. Fastmail knows whether the address exists, and its answer reflects that knowledge. A nonexistent address on a Fastmail hosted domain is rejected with a 550, and Mailthentic reports it as invalid with confidence. An existing mailbox is accepted at RCPT TO, and because the accepting host is authoritative rather than a relay, that acceptance is meaningful evidence rather than a shrug.

Catch-all and aliases

Fastmail is not catch-all by default, which is why an individual mailbox can genuinely be confirmed on most Fastmail domains. However, Fastmail gives customers on custom domains the option to enable a catch-all address that receives anything sent to the domain. Where a customer has switched that on, the domain behaves like any other accept-all domain: every local part is accepted, including nonsense, and no individual mailbox can be confirmed. Mailthentic detects this the same way it detects it anywhere else, by probing with a randomly generated address that could not plausibly exist. If that address is accepted, the domain is flagged accept-all and the individual result is reported as ambiguous rather than valid.

Aliases and masked addresses are a related nuance. A Fastmail alias is a real, deliverable address that routes to an existing mailbox, so it is correctly reported as valid. It is worth remembering that a masked address can be disabled by its owner at any time, which is precisely why they exist, so a valid verdict on a masked address today is not a permanent guarantee of deliverability tomorrow.

Rate limiting and greylisting

Fastmail applies connection and rate controls to protect its platform, and unfamiliar or high volume sources may receive temporary responses in the 421, 450 or 451 range. These are deferrals, not rejections, and it would be wrong to treat them as invalid addresses. Mailthentic classifies them as temporary, backs off, and retries the probe on a delay. Throttling is applied at the MX cluster level, so all Fastmail hosted domains in a list, including custom domains, are paced against the shared Messaging Engine infrastructure rather than probed independently and simultaneously.

What a result here genuinely means

Fastmail is one of the better providers to verify against. Because the responding host is authoritative for the mailbox, a valid verdict on a non catch-all Fastmail domain is strong evidence that the mailbox exists, and an invalid verdict is a definitive rejection. The one case where certainty disappears is a custom domain with catch-all switched on, and Mailthentic will tell you plainly when that is what it found rather than dressing an accept-all up as a confirmed address.

Trust the verdicts, with one caveat

On a standard Fastmail domain, a valid result is genuinely useful and an invalid result is safe to act on. Remove invalid addresses permanently. The caveat is the custom domain with catch-all enabled: if Mailthentic reports accept-all, no individual mailbox on that domain has been confirmed, and you should treat those addresses with the same caution you would apply to any other accept-all domain.

Handling accept-all Fastmail domains

• Do not delete them automatically. Accept-all does not mean bad, it means unproven.
• Prioritise contacts with prior engagement, since real opens and clicks are stronger evidence than any handshake.
• Send in small batches first and watch the bounce rate for that segment specifically before committing to a full send.
• Feed every bounce straight into suppression.

Respect the audience

Fastmail users pay for their email and they chose an independent provider on purpose. They tend to be technically literate, they read headers, and they are quick to report unsolicited mail. That makes complaint risk real, and complaints from this audience are not the kind you can absorb quietly. Only mail people who genuinely opted in, make the unsubscribe link obvious and one click, and honour it immediately.

Masked and alias addresses

If a subscriber signed up with a masked address, that address is real and deliverable but it exists so the owner can switch it off at will. A sudden hard bounce on a previously good masked address is usually a deliberate act, not a data quality problem. Suppress it and do not attempt to reach the same person by guessing at their underlying mailbox.

Protect deliverability

• Publish and maintain SPF, DKIM and DMARC. Fastmail checks authentication and weighs it in filtering.
• Slow down when you are told to. A 421 or 451 from Messaging Engine is a request to back off, and hammering it damages the reputation of your sending IP.
• Warm new sending domains gradually rather than launching at full volume.
• Keep list hygiene continuous. Re-verify addresses that have been dormant for months rather than assuming a verdict from last year still holds.

Suppress promptly

Hard bounces, spam complaints and unsubscribes should all result in immediate, permanent suppression. On a paid, privacy-focused provider, the cost of getting this wrong is disproportionate to the size of the audience, and the reputational damage carries over to your sending on every other platform.