It's important for users created automatically by LDAP, CSV import, or by SMTP to log into the Control Panel at least once, for them to be able to log into the Continuity Service's webmail. Otherwise, they'll see an "invalid username/password" error.
Manually created users won't need to log into the CP beforehand, they will be able to log into the continuity service straight away.
The reason for this is that the continuity service must compare the username/password entered in the continuity service webmail portal with the username/password cached in the control panel.
Please understand that, during a continuity service event, where the customer's destination server is unreachable, we may also be unable to reach your LDAP server for login credential verification, so we rely on our cached credentials.
For SMTP generated users, the password is randomized in the control panel (since the user is automatically generated). Your users will not know what the randomly generated password is. So they will need to request a password reset from the control panel so they can manually set their password. This needs to be done before a continuity event because if their destination server is unreachable they won't be able to receive the password reset token.