Mozilla Thunderbird is an open source, multiplatform and extensible email client. It has been around under free license and stable since 2004 and now at versions 10 and 11 is a fully featured and mature product. All the features that you would expect from a modern email client are present: HTML support, multiple account support, whereby you can keep accounts separate or merge them together, Web mail, POP, IMAP and Microsoft Exchange server support, calendar, address book.
