getting rid of the passwd file unknown user messages

I have configured a userdb file, so I can test with some accounts and override settings. But it is a bit annoying getting unkown user messages for the majority of accounts not in this userdb. Is there a way to skip this reporting?

dovecot: auth: passwd-file(aaaaaaaaaaaa,X.X.X.X,<lGapvOzGS5tNPz8/>): unknown user