Geary is an IMAP mail client for GNOME 3.
Integrated email and groupware client.
Thunderbird is a free cross platform e-mail application that is easy to use and customize.
Mutt is a text based email client to be used inside of a terminal emulator.
Evolution is a personal information management application that has calendar, email and contact support.
Claws Mail is a cross platform e-mail client based on GTK+.