This means that the Exchange server has been configured to require a secure authentication method. To log in, you either have to:
1) Try authenticating using NTLM by caling
Imap object's `Login(userName, password, ImapAuthentication.Ntlm). Please note that Rebex Mail 1.0.3588.0 and later attempts this automatically. In Rebex Mail 1.0.3723.0 and later, you can try GSSAPI/Kerberos as well (ImapAuthentication.GssApi).
2) Establish an SSL-protected connection to the server before authenticating (needs Rebex Secure Mail or Rebex Secure IMAP).