Evolution is the GNOME DesktopEnvironment's MailClient/PIM. It was written by Ximian (now part of Novell) as an equivalent to MicrosoftOutlook, for Linux and other Unix platforms. They make a plugin that lets Evolution talk to MicrosoftExchange servers. From version 2.3.6 on it supports inline PGP signed/encrypted messages thanks to some patches by MattBrown.
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