What Is Greylisting in Email Delivery?
Greylisting is a spam-prevention technique where a receiving mail server temporarily rejects an email from an unrecognized sender with a "try again later" response, then accepts it on a retry after a short delay - because most spam senders never retry, but legitimate mail servers do.
How greylisting works
When a receiving server sees mail from a sender/IP combination it hasn't seen before, it returns a temporary SMTP error (typically 450 or 451) instead of accepting or rejecting outright. Legitimate mail servers are built to retry after a delay (often minutes), while most spam-sending infrastructure does not bother retrying, so it never gets through.
Why greylisting matters for email verification
A verification check that only attempts a single SMTP connection can be fooled by greylisting into reporting "unknown" or a temporary failure for a perfectly valid address, simply because the receiving server hasn't "warmed up" to the sending IP yet.
How RocketVerifier handles greylisting
RocketVerifier's verification fleet retries SMTP checks with appropriate delays and uses a distributed set of sending IPs with established reputation, reducing false "unknown" results caused by greylisting.
FAQ
Does greylisting stop spam completely?
No - it's one layer among many (alongside SPF/DKIM/DMARC, content filtering, and reputation scoring), but it remains effective against low-effort spam infrastructure that doesn't retry.
How long does a greylisting delay typically last?
It varies by mail server configuration, but delays of a few minutes to an hour are common before a retry is accepted.
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