What do you mean with non-standard default mail format? Most mail I receive from Claws users seems perfectly fine, which is something you can't say for most oth

What do you mean with non-standard default mail format? Most mail I receive from Claws users seems perfectly fine, which is something you can't say for most other popular MUAs. Outlook and certain web mailers probably being the most egregious examples, only topped by some (thankfully rare) Android vendor mobile apps. The TLS setup certainly sucks but OTOH, the actual distribution archives are all cryptographically signed. Most users don't get Claws Mail from the website anyway, but use their distro packages, whose maintainers have hopefully verified the signatures before importing a new release.