Mozilla e-mail, calendar, rss and usenet client
Mozilla Thunderbird is a messaging client including support for email (POP3 and IMAP), RSS feeds, chat (IRC, XMPP and others), and newsgroups (NNTP). It has features supporting user privacy and safety (remote image blocking, protection against some forms of phishing, "learning" spam filter), and offers easy organization of emails with tagging and virtual folders. It also has calendar/groupware features that can handle events, event invitations and tasks in multiple calendars. It supports local calendars, CalDAV and plain .ics files on CardDAV and WebDAV. More features can be added by installing extensions.
Homepage: https://www.thunderbird.net/