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 [email protected], 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.