Timing and protocol defects
From ASRG
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.
