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Mail (also known as Mail.app or Apple Mail) is an email program included with Apple Inc.’s OS X operating system. Originally developed by NeXT as NeXTMail, a part of their NeXTSTEP operating system, it was adapted to become OS X’s Mail application following Apple’s acquisition of NeXT. The current version of Mail utilizes the SMTP for message sending, POP3 and IMAP protocols for message retrieval and S/MIME for end-to-end message encryption. It also supports Yahoo! Mail, AOL Mail, Gmail, iCloud (formerly MobileMe) and Exchange. iOS features a mobile version of Apple Mail with added Exchange ActiveSync support, though it is still missing the functionality of attaching files to reply emails. Mail has often been praised for its combination of simplicity and extensive functionality . As of April 2014, the iOS version of Mail is the most popular email client in the world by market share.