Definition
Unknown result: A verification outcome when the mail server gives no clear answer, for example after a timeout, greylisting or a blocked probe. RocketVerifier never charges credits for unknown results.
Why email verification returns unknown
Mail servers protect themselves. Greylisting defers first contact, rate limits slow repeated checks, and some servers block connections from IPs they do not know. A DNS failure or an unreachable server also leaves nothing to read.
There is one more case: the server accepted the address, but the random-address test that rules out catch-all was blocked. Without that test, a yes means nothing, so the correct result is unknown.
What to do with unknown email addresses
Retry them later. Greylisting and temporary blocks often clear within hours, and a second pass turns many unknowns into clear results.
If they stay unknown, keep them out of your main send. Mail them separately in a small batch, ideally where you have another signal such as a recent reply, and suppress any that bounce.
Why unknown is better than a guess
A verifier that guesses deliverable when a server will not answer sends you bounces. One that guesses undeliverable deletes real customers from your list.
Calling it unknown tells you exactly where the risk is, and lets you decide what to do with it.
How RocketVerifier handles it
Our unknown sub_status tells you why: dns_error, greylisted, blocked, smtp_timeout, no_connect or smtp_unavailable. Greylisted checks are retried automatically, up to 3 times about 10 minutes apart, and MX hosting alone, including Google Workspace and Microsoft 365, is never treated as deliverable. Unknown results are never charged, so retrying them costs nothing until one comes back with a clear answer.