We actually have such rules:
list "HORDE" addr { 10.0.100.41 10.0.100.42 10.0.100.43 10.0.100.44 10.0.100.45 }
ratelimit "MAILS_PER_HOUR" rcpt 200 / 1h key "%f"
racl blacklist list "HORDE" ratelimit "MAILS_PER_HOUR" msg "Quota Exceeded"
But we only can use %f and other parameters specified at the manual.
The point is that we want to use as a parameter, the SASL authenticated user, and I don't know if we can do it and how.
Thanks