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K9 mail

Someone says it’s one of the best android App.

K-9 Mail is an open source email client with support for multiple accounts, search, IMAP push email, multi-folder sync, flagging, filing, signatures, BCC-self, PGP/MIME & more! 

K-9 supports IMAP, POP3 and Exchange 2003/2007 (with WebDAV).