The metacharacters $?x? in a mapping table entry cause the entry to “succeed” x percent of the time; the rest of the time, the entry “fails” and the output of the mapping entry's input is taken unchanged as the output. (Note that, depending upon the mapping, the effect of the entry failing is not necessarily the same as the entry not matching in the first place.) The x should be a real number specifying the success percentage.
For instance, suppose that a system with IP address 126.96.36.199 is sending your site just a little too much SMTP email and you'd like to slow it down; you can use a PORT_ACCESS mapping table in the following way. Suppose you'd like to allow through only 25 percent of its connection attempts and reject the other 75 percent of its connection attempts. The following PORT_ACCESS mapping table uses $?25? to cause the entry with the $Y (accept the connection) to succeed only 25 percent of the time; the other 75 percent of the time, when this entry fails, the initial $C on that entry causes the MTA to continue the mapping from the next entry, which causes the connection attempt to be rejected with an SMTP error and the message: Try again later.
PORT_ACCESS TCP|*|25|188.8.131.52|* $C$?25?$Y TCP|*|25|184.108.40.206|* $N45s$ 4.40$ Try$ again$ later