How to verify a Fastmail address

Fastmail's servers will discuss a recipient, but only with a checking server they trust. Like most providers, they screen the connecting IP against reputation lists first. RFC 3463 puts x.7.x codes in the security and policy class, so a 5.7.1 naming a blocklist is a refusal of the checker, not a missing mailbox.

This is the most common way verifiers produce false invalids on small, well-run providers. A tool that maps every 5xx to "invalid" would report real Fastmail users dead whenever its own IP is blocked. A careful verifier checks the enhanced code and returns unknown for policy rejections. If you run your own checks, look up your server's IP first with the blacklist checker.

Fastmail supports two ways to create disposable variations of one address. Plus addressing works as elsewhere, while subdomain addressing moves the tag in front: [email protected] lands in the same account as [email protected]. Subdomain addresses defeat simple de-duplication that only strips plus tags, so if Fastmail users matter to you, normalise both forms before counting unique people.

Fastmail has no free plan; its trial page says trials last up to 30 days and accounts lose the ability to send and receive if they are not paid for, then are disabled and eventually deleted. Paid accounts that lapse follow the same path, and Fastmail's help says that once an account is deleted "the username and any addresses on the account can be re-used."

That last point matters for list hygiene. A Fastmail address from a trial signup years ago may now belong to someone else entirely, who never agreed to hear from you. Re-confirm consent with old Fastmail contacts rather than relying on an old opt-in, and treat repeated bounces as final.