What is greylisting

Greylisting is an anti-spam technique where a mail server temporarily rejects incoming mail from an unknown sender. The assumption is that a legitimate mail server will retry delivery after a few minutes, while a spam bot will not.

How it works

When an unknown sender triplet — sending IP, sender address, recipient address — is seen for the first time, the receiving server returns SMTP code 450, a temporary failure. SMTP requires the sending server to queue the message and retry later. The greylisting filter typically accepts the retry after 5 to 15 minutes and records the triplet so future mail from the same sender passes without delay.

Most spam bots skip the retry entirely. That is the whole bet greylisting makes, and it works well enough that the technique cuts a meaningful share of junk traffic at very low compute cost.

Who uses it

Greylisting is built into Postgrey (for Postfix), SQLgrey, and milter-greylist, and a number of commercial anti-spam appliances include it as well. Some hosting providers enable it by default. Large mail services like Gmail or Outlook do not use it — they have enough other filtering signals that the delay trade-off is not worth it.

Effect on email validation

For email verification services, greylisting is a real problem. During SMTP validation, the server responds 450 to the RCPT TO command. Without a retry, a naive validator reads that as the address not existing — a false negative.

Validators that handle this properly implement retry logic: after receiving a 450, they wait and try again. Some also keep a cache of known greylisting servers so they can schedule the delay from the start rather than discovering it mid-check.

How uChecker handles greylisting

uChecker detects 450 responses and retries the check automatically after the appropriate delay. This reduces false negatives on servers that use greylisting and gives accurate results where simpler validators would just give up.