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:<aa@your.domain>
550 5.1.1 <aa@your.domain>... User unknown
RCPT To:<ab@your.domain>
550 5.1.1 <ab@your.domain>... User unknown
RCPT To:<ac@your.domain>
550 5.1.1 <ac@your.domain>... User unknown
RCPT To:<ad@your.domain>
550 5.1.1 <ad@your.domain>... User unknown
RCPT To:<ae@your.domain>
250 2.1.0 <ae@your.domain>... Recipient ok
RCPT To:<af@your.domain>
550 5.1.1 <af@your.domain>... 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 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.

With V8.12 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 (V8.12 and later)
-OBadRcptThrottle=num                   ← command line (V8.12 and later)
define(`confBAD_RCPT_THROTTLE',`num')   ← mc configuration (V8.12 and later

Here, num is a textual representation of a positive integer. If num is negative, non-numeric, or zero (the default), bad RCPT To: commands are accepted without penalty. If num is ...

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