sendmail, 4th Edition

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.^[136] 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 V8.13 sendmail to combat slamming. You use the FEATURE(greet_pause) like this:

FEATURE(`greet_pause', `ms_pause')

The FEATURE(greet_pause) takes a single argument, an integer representation of the number of milliseconds to wait before sendmail may send its initial greeting. The ms_pause sets the default wait (we cover this shortly). If ms_pause is missing, no default is set. If ms_pause is greater than five minutes, the wait is silently truncated to five minutes.^[137]

If sendmail detects input from the client during this wait, that input is interpreted as an indication of slamming. If ...