On IMAP mailservers, apple’s iPhone often complains that “Mobile mail can’t verify the server’s identity”. Unfortunately, this is because the iPhone currently has no version of iOS that will allow you to permanently accept self signed certificates.
On most modern browsers you can permanently store the certificate so that you can trust your mailserver domain, but until this arrives on the iPhone we are left with either disabling SSL or living with the “Continue” alert. I would always recommend living with the alert because mobile data needs to be secured, especially at public WiFi spots.