Claws Mail is an e-mail client (and news reader) based on GTK+

Currently, many features are supported including POP3, IMAP, NNTP,
multiple accounts, threading, filtering, MIME attachments, APOP, SMTP
AUTH, SSL, IPv6, GnuPG, internalization, and more.

In addition to the above, Claws Mail has a lot of extra features,
e.g. a plugin mechanism, a new filtering/processing mechanism,
extra folder properties and much more...

WWW: https://www.claws-mail.org/