CB does not pick or choose who it sends emails to, take this up with your mailserver; it's possible they'd filtering emails. If your mailserver is functioning then it won't stop CB from sending emails, it has no code in place to selectively pick who it chooses to email.
To resend a confirmation email a user simply needs to attempt to login and it will send them a new confirmation email.
It's also possible users are providing fake emails which would cause them to never arrive anywhere. Again, please examine your mailserver logs (contact your host for this).