IPhone 4S announced iPhone 5s and iPhone 5c. This was unusual for Apple as an upper case 'S' had been used since the introduction of the iPhone 3GS in 2009. On September 9, 2014, the iPhone 4S was officially discontinued following the reveal of the iPhone 6 and the iPhone 6 Plus. However, 4S production did continue for developing markets until February 17, 2016. On June 13, 2016, following the release of iOS 10, Apple dropped support on the iPhone 4S, making iOS 9.3.5 the last version available for the device. The runs iOS, Apple's mobile operating system. The user interface of
IPhone only available on jailbroken iPhones. The iPhone also features an email program that supports HTML email, which enables the user to embed photos in an email message. PDF, Word, Excel, and PowerPoint attachments to mail messages can be viewed on the phone. Yahoo! offers a free push-email service for the iPhone. IMAP (although not Push-IMAP) and POP3 mail standards are also supported, including Microsoft Exchange and Kerio Connect. In the first versions of the iPhone firmware, this was accomplished by opening up IMAP on the Exchange server. Apple has also licensed Microsoft ActiveSync and supports the platform (including push email)