Mail in Apple iOS before 8.4 and OS X before 10.10.4 allows remote attackers to trigger a refresh operation, and consequently cause a visit to an arbitrary web site, via a crafted HTML e-mail message.
http://lists.apple.com/archives/security-announce/2015/Jun/msg00001.html
http://lists.apple.com/archives/security-announce/2015/Jun/msg00002.html
http://support.apple.com/kb/HT204941