The Bat!
Link
https://www.ritlabs.com/en/products/thebat/
Excerpt
The Bat! is a secure desktop email client for Windows, built to protect your correspondence against third-parties.
Review
I have been using The Bat! on and off since 2012 and probably years before that, but that’s how far back I have saved my licenses.
Positive
- Low system requirements, snappy, can handle large mailboxes easily
- Supports PGP and S/MIME
- Portable mode (e.g. run from a flash drive)
- Extensive filtering and automation (rules) options
- Very customizable
- Usable HTML view, not affected by some other mail client’s security issues or bad CSS support
- Integrated RSS reader
- Purchase supports developers in one of Europe’s poorest countries
Negative
- Only supports Windows
- Not open-source (could cease to be developed)
Especially for version 9 (I don’t have enough experience with version 10 yet)
- It was a bit unstable
- Gmail authentication was bugged