This is quite an old post, but as it is the first link that I found when search for more information about this error, I thought I would post here.
The error that is being posted to your logs indicates that someone tried authenticating with postfix but their details were not found. If you are get lots of this, then there is probably a spambot (most likely several) trying brute force email accounts so that it can send spam via your server.
It is nothing to overly concerned about, but if it is causing you problems (or you just want to put a stop to it) - you may want check out something like
Fail2Ban.