How to Verify a Fastmail Address (fastmail.com and Domains Hosted on Fastmail)
Fastmail addresses are usually checkable, because Fastmail's servers commonly refuse an address that does not exist. In our October 2026 test of 51 consumer providers, fastmail.com rejected a made-up address in both rounds. A verifier's valid or invalid answer therefore comes from Fastmail. The full table is in how 51 mailbox providers answered a verification check.
The part that is easy to miss: custom domains
Most Fastmail users you meet in B2B lists are not on fastmail.com. Many people and small teams host their own domain on Fastmail. In the MX record, those domains commonly point to hosts under messagingengine.com. Run your domains through the bulk MX lookup to count them. Reading the provider behind a domain is covered in how to check which email provider a prospect uses.
What each result means
| Result | Reason you will see | What it means |
|---|---|---|
| Valid | Mailbox exists | Fastmail accepted this exact address |
| Invalid | Mailbox not found | Fastmail said it does not exist. Remove it |
| Risky | Accepts all addresses | A catch-all or wildcard alias is on |
On a domain with no catch-all, valid and invalid are reliable signals, because the server has shown it will say no.
Aliases and catch-all on Fastmail
Fastmail supports aliases, including a wildcard that catches everything sent to a domain. If a domain owner has turned that on, the server accepts any address at that domain. Then "valid" proves nothing, and the honest result is risky with "Accepts all addresses". Treat it as a catch-all. See what a catch-all domain is and how to send to catch-all emails.
Aliases have another effect. One person can have many addresses that all land in one inbox. If your list has several for the same domain, you may be mailing one person many times. Deduplicate by domain before sending.
What a check cannot tell you
- Whether the person reads a given alias.
- Whether the mailbox is still open when you send.
- Whether Fastmail will accept your message, which depends on authentication and reputation.
- Anything about domains with a catch-all on.
Verify before you send
A small test batch on your own list, then a full run, is the cleanest way to confirm. Try a few on the verify page: the first 100 email checks are free with a Google sign-in, no card. Each comes back valid, risky or invalid with a reason, and nothing is sent to anyone on the list. For larger files, see bulk email verification, list cleaning and pricing.
Common questions
Can you verify a Fastmail address without emailing it?
Usually, yes. In our October 2026 test Fastmail refused a made-up address at the recipient step in both rounds, so the answer comes from Fastmail and an unknown address returns Mailbox not found (invalid).
How do I recognise a domain hosted on Fastmail?
Look up its MX record. Fastmail's mail servers sit under messagingengine.com, and domains hosted there commonly point to hosts on that name.
Why does a Fastmail-hosted address come back as Accepts all addresses?
Fastmail lets an account owner use a wildcard or catch-all alias for a domain. When that is set, every address at the domain is accepted and the check cannot separate real from invented. The result is risky.
Is a Fastmail address likely to be a business contact?
Sometimes. Many individuals and small teams run their own domain on Fastmail, so a company-looking address may be a one-person business.
Do I need to treat Fastmail differently from Gmail?
Not in how the check works. Both commonly answer honestly at the recipient step. The difference is mostly in the audience and the chance of aliases.
Check your list before you send
Real mailbox checks over SMTP, and nothing is sent to the address. Sign in with Google and your first 100 email checks are free, no card. Plans for bigger lists are on the pricing page.