When a user types an invalid password, “authentication” failure is logged, as opposed to a “user not found” message. The message “user not found” is the text passed to the client for security reasons, but the real reason (invalid password) is logged.
[30/Aug/2004:16:53:05 -0700] vadar imapd[13027]: Account Notice: badlogin: [192.18.126.64:40718] plaintext user1 authentication failure |