Hello Alastair,
As a matter of fact, I am considering to separate the logs - have different file for different services, but I have a new release almost ready, so, probably, I will be adding it to next release.
Failed authentication attempts are very common, especially with SMTP - spammers are trying to hack your passwords, but if you have auto lockout feature enabled, you should not be worried about it, because your server will handle things.
My server log is full of those, and I am logging them to Windows application logs too.
Archie