- aerc: MIT, Go
- alpine: Apache 2.0 (parts 4-clause BSD), C
- claws mail: GPLv3, C
- Gnus: LGPLv2+, elisp
- KMail: GPLv2 (parts GFDLv1.2, parts LGPLv2.1), C++
- mutt: GPLv2, C
- SquirrelMail: GPLv2, PHP
Side note:
> (context: ProtonMail) It's also recommended that you visit Settings → Account → Identity and remove the default email signature.
It bears noting that free accounts on ProtonMail cannot remove the ProtonMail advertisement in the signature, cf. [1].
[1] https://www.reddit.com/r/ProtonMail/comments/5ageqz/question...