As far as I can see only gmail offers that feature.
Safe-mail is the only option atm i think. So we have to use it until it's fixed. I can confirm email messages in multithreading mode. The email confirmation part works really good for me, but only with safe-mail.Don't come with this "use this email or that email" crap. I want to use the provider I want to use without ZP deleting the e-mails and that's it. This is a serious weakness in the software that prevents multithreading on signup.