24.9.9 BadRcptThrottle
Slow excess bad RCPT To: commands V8.12 and later
One method used to gather addresses for spamming is to misuse the RCPT To: command.
To illustrate, consider the following fragment of an SMTP session:
RCPT To:
550 5.1.1 ... User unknown
RCPT To:
550 5.1.1 ... User unknown
RCPT To:
550 5.1.1 ... User unknown
RCPT To:
550 5.1.1 ... User unknown
RCPT To:
250 2.1.0 ... Recipient ok
RCPT To:
550 5.1.1 ... User unknown
Here, some other site has connected to your sendmail and started sending bad RCPT To:
commands for a series of possible usernames. These are alphabetical, but other such abuses
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980 | Chapter 24: The O (Options) Configuration Command
might be based on lists of common names. Whenever sendmail replies with a 250, the other
site knows that address is good, and adds it to its list of spam addresses.
WithV8.12 and later sendmail, it is possible to impose a penalty on sites that send too
many bad RCPT To: commands. You do that by defining the BadRcptThrottle, like this:
O BadRcptThrottle=num ?†? configuration file (V8.
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