SimpleVerifier Team · Reviewed
Why do email verifiers return unknown or unverifiable?
Unknown means the verifier could not get a clear answer from the recipient's mail server: it refused the connection, timed out, deferred with a temporary error, or hides whether mailboxes exist. It does not mean the address is bad. Retry later, and treat the address like a risky one until a check confirms it.
Common causes
The usual causes, in the order we see them.
- The provider does not reveal mailbox status (Microsoft consumer domains and iCloud in our tests).
- The server accepts every address (Yahoo and AOL, and ISPs they host).
- The server refused the verifier's connection: in our study, United Internet (GMX, web.de), T-Online, Comcast and Bigpond refused our check.
- A temporary 4xx response such as greylisting, which asks the sender to try again later.
How SimpleVerifier handles it
These all return risky with the reason named, and never valid. A retry result means the server did not answer and a later check may succeed.
Related questions
- What is greylisting?
- A server temporarily rejects mail from a sender it has not seen, expecting a legitimate sender to retry. A verifier sees that as a deferral, not an answer.
Sources
- How mailbox providers answer verification checks, 2026 (SimpleVerifier study, 51 providers, CC BY 4.0)
- RFC 5321: Simple Mail Transfer Protocol
Figures marked as ours come from SimpleVerifier’s own dated tests on made-up addresses, published with their data on the research page. No customer data is used, and we publish no accuracy percentage because we have no independent measurement to back one.
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