iPhone Mail in Apple iPhone OS, and iPhone OS for iPod touch, does not validate X.509 certificates, which allows man-in-the-middle attackers to spoof arbitrary SSL e-mail servers via a crafted certificate.
https://exchange.xforce.ibmcloud.com/vulnerabilities/53234
http://www.securityfocus.com/bid/36370
http://www.securityfocus.com/archive/1/506428/100/0/threaded