# Fastmail: MX records and email verification

How Fastmail-hosted domains look in DNS, how Fastmail answers an SMTP verification handshake, and which verdicts to expect for Fastmail addresses.

Updated: 2026-09-03

Fastmail is an independent mailbox provider with a large base of custom-domain customers. Its infrastructure runs under the `messagingengine.com` name, which is why Spaw reports it as `mx_provider: "fastmail"` even though that hostname never appears in a Fastmail address.

## How Fastmail shows up in DNS

Fastmail's DNS guide asks custom domains to publish `in1-smtp.messagingengine.com` at priority 10 and `in2-smtp.messagingengine.com` at priority 20. Fastmail's own domains (fastmail.com, fastmail.fm and the many alternative domains it offers) use the same hosts. Spaw matches the `.messagingengine.com` suffix. Fastmail is a mailbox host: the MX target stores the mail, `mx_found` is true and `mx_implicit` is false.

## How Fastmail answers a verification handshake

Fastmail typically rejects unknown recipients at `RCPT TO`, so most mailbox checks settle on the first attempt. Two Fastmail features change that picture for some domains. Subdomain addressing lets any user receive mail at `anything@user.fastmail.com`, which is effectively a per-user catch-all on that subdomain. And custom domains can be configured with a catch-all address, in which case the inbound server accepts every recipient and the domain answers as catch-all.

## What that means for your verdicts

Fastmail addresses mostly come back `deliverable` with `mailbox_exists: true` or `undeliverable` with `reason: mailbox_not_found` and `smtp_reason: no_mailbox`. Catch-all domains answer `risky` with `reason: catch_all`; Spaw remembers the domain for seven days and returns `mailbox_confidence`, which includes the ten-point bonus for an established provider. Fastmail is not in the consumer username rule set, so `invalid_local_part` does not apply; plus-addressing is supported and reported as `is_alias: true`.

Fastmail users are often technical and privacy-conscious, and Fastmail's masked-email feature creates single-purpose addresses on domains such as `fastmail.com` subdomains that stop accepting mail once the user disables them. Such an address is real until it is not; a later lookup answers `mailbox_not_found` and the address joins your suppression list.

## SPF and DMARC at Fastmail

Fastmail's manual DNS guide publishes `v=spf1 include:spf.messagingengine.com ?all` for SPF, DKIM CNAME records for the `fm1`, `fm2` and `fm3` selectors, and suggests starting DMARC at `v=DMARC1; p=none;` so you can observe before enforcing. Note the neutral `?all` in Fastmail's default: it is a permissive record, and Spaw still reports `has_spf: true` for it because the record exists.

## Practical notes for sending to Fastmail users

Fastmail's filtering weighs authentication and reputation in the usual way and applies user-defined rules heavily, so a properly aligned sending domain is the main requirement. Expect masked and single-purpose addresses on lists collected from technical audiences and re-verify those lists on a schedule; a monitor in the Spaw dashboard does that for you and alerts when deliverability decays.

For a live look at any Fastmail-hosted domain, run it through the free MX lookup and the SPF and DMARC checker.

Reference: https://spaw.co/email-providers/fastmail