If the HELO/EHLO greeting is rejected, the client will receive a permanent rejection like
the following:
550 5.7.1 bogus HELO name used: client??™s bogus hostname here
This is the Title of the Book, eMatter Edition
Copyright ?© 2007 O??™Reilly & Associates, Inc. All rights reserved.
7.6 Spam Suppression Features | 293
This FEATURE(block_bad_helo) is implemented as the last check in the check_rcpt rule
set (?§7.1.3 on page 257).
7.6.3 FEATURE(greet_pause)??”V8.13 and Later
Slamming is a technique used by some senders of spam email. It allows spamming
machines and hijacked proxies to send a great deal of spam email very rapidly, without
the need to monitor for rejections.* This is a boon to spam-email companies, but
a bane to those who resent that behavior.
To slam, a spammer first opens a connection to the SMTP server (in our case, a listening
sendmail daemon). Normally, the sending client will not send anything to the
server until the server issues its initial greeting:
220 mail.example.com ESMTP Sendmail 8.14.1/8.14.1; Thu, 13 Aug 2007 07:45:41 -0800
(PST)
With slamming, however, the client does not wait for the initial greeting. Instead, the
offending client sends its entire SMTP message all at once, then disconnects, before
the server (sendmail) has a chance to review the message??™s contents.
The FEATURE(greet_pause) was added to V8.
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