Thunderbird

Thunderbird is a desktop email client that keeps messages on your own machine. It handles several accounts at once over IMAP, POP3 and SMTP, with a unified view across mailboxes, message filters, saved searches and a tagging system that survives across folders.

Mail is only part of it. The calendar is built in, reads and writes CalDAV and iCalendar, and sits next to an address book that syncs over CardDAV. Feeds and newsgroups are read in the same window. End-to-end encryption is available through OpenPGP without an add-on, and S/MIME is supported for organisations that use certificates.

The application is maintained by MZLA Technologies, a subsidiary of the Mozilla Foundation, and is funded mainly by user donations. Builds are published for Windows, macOS and Linux, with the extended support release available for people who prefer fewer, larger updates.