What is tarpitting?
Also called: SMTP tarpit, teergrube
Tarpitting is a mail server deliberately slowing its replies to a client it suspects of spam or address harvesting, sometimes by many seconds per command. Real senders tolerate the delay; bulk spam tools and verification probes, which open many short connections, are slowed down or time out.
It is typically triggered by behaviour rather than by a single message: many recipients in one session, many rejected RCPT TO commands in a row, or a connecting IP with a poor reputation. The server is not refusing anything, it is making each step expensive.
For senders it shows up as slow delivery to one domain. For verification it shows up as probes that never finish, which means the mailbox question goes unanswered. That is an inconclusive result, not evidence that the mailbox is bad.
The fix on the verification side is to keep a low, steady rate per destination and to stop rather than hammer a server that has started slowing down.
How SimpleVerifier handles it
Probes run at a steady, capped rate, and a probe that times out returns risky · Mailbox check unavailable rather than a guess.
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By SimpleVerifier Team · Last reviewed 2026-10-13 · All 50 glossary terms