Timing and protocol defects

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Anti-spam technique: Timing and protocol defects
Date of first use:
Effectiveness: High
Popularity: Medium
Difficulty of implementation: Low
Where implemented: MTA
Harm: Low


SMTP requires that clients wait for a 2xx response to the HELO or EHLO command before proceeding. One ant-spam technique, known as a premature pipeline check, detects extra data in the input buffer prior to the server sending the HELO/EHLO response. If such data is found, the client has failed the test. The server might then reject the message, close the connection, blacklist the client, etc.