[exim] catching brute foce smtp auth - what condition?

Hello.

How to catch brute foce smtp auth attempts only? (== bad login or
password provided)

Condition like:

${if eq{$authentication_failed}{1}}

doesn't work because it also catches cases where client cancelled smtp
auth attempt (rfc2554 and "*").


Exim internally sees difference:

535 Incorrect authentication data
501 Authentication cancelled

Ideas?

--
Arkadiusz Miśkiewicz, arekm / ( maven.pl | pld-linux.org )