This error is pretty non-descriptive, but one of the reasons this error will occur is the mail server is configured to use APOP password authentication. With APOP, the password is sent to the mail server in an encrypted form rather than in clear text.
Currently, 1.2.x versions of CyberDog do not support APOP password authentication. The user must ask their mail administrator to disable the APOP requirement for their account.
NOTE: CyberDog 2.0, when released, will support APOP password authentication, as demonstrated in the currently available CyberDog 2.0b1 beta software. Users can download this version from the CyberDog Home Page at
http://cyberdog.apple.com/. Note that beta versions of software are not supported by AppleAssist.