An SMTP server has introduced a deliberate pause before sending the greeting banner to the client. In client server architecture, the client is required to wait and it is not supposed to send the message to the server unless it receives the banner. However, many spam-sending machines begin to send out messages before the banner is received and the servers can detect this and drop the connection.

However, some legitimate sites also fall prey to this mechanism. The mechanism also has a tendency to interact badly with sites performing callback verification, since common callback verification sites have timeouts much shorter than those mandated by RFC 5321.

Callback verification, also known as callout verification or Sender Address Verification, is a technique used by SMTP software in order to validate e-mail addresses. The most common target of verification is the sender address from the message envelope (the address specified during the SMTP dialogue as “MAIL FROM”). It is mostly used as an anti-spam measure.

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