IMHO, it's also the best if you're using Gmail: Google has been breaking its UIs for years (both on the app and on the web), especially for people like me who try to use proper quoting and signatures, or to send plain text emails. Apart from some glitches with reflow, K-9 does that very well even with Google as email provider.
I can't compare against Microsoft products since I've never used them on mobile, but seeing how horrible Outlook is on the desktop for plain text emails (and that's not an hyperbole), I bet K-9 is better as well.
the only way to make it usable again is to switch to the plain html mode, that uses no javascript, loads very fast, and reminds me a lot of the first gmail ui, back from ~15 years go.
https://chrome.google.com/webstore/detail/simplify-gmail/pbm...
Injecting non-google code inside my gmail window sounds like a very dumb idea to me, honestly.
I already trust google very little and keep my gmail inbox because I'm basically being coerced/forced to (no gmail = no apps on android)... The last thing I want is adding another entity to the picture.
This helps avoid app-specific passwords when you use 2FA, and lets users use their regular sign-in flow (which could include enterprise SSO, TOTP, U2F key etc).
I imagine that there's the ability for Google to set some cookies as part of that process, although knowing Apple, would not be surprised to learn they had sandboxed that instance of the browser, to prevent cookies persisting into regular Safari.
So I wouldn't worry about it.
K9 originated in the old android AOSP email app, and I think the developer of FE wanted to cut loose from that and build it according to newer design patterns.
As a caveat here, at least in the non-beta version, support for PGP signatures is intentionally crippled because the original author dislikes them and apparently wants to push encryption (eg, https://github.com/k9mail/k-9/issues/2375), ignoring everyone who points out that this feature may be important or required by policy for them. By default, PGP signatures on unencrypted emails are silently hidden and the emails are displayed as being unsigned. There is no way to sign emails by default, and trying to sign an email results with a popup telling you not to.
* https://k9mail.app/2016/11/24/OpenPGP-Considerations-Part-I....
I think there are implementations out there that don't let you send encrypted but unsigned messages (anonymous encrypted). This stuff seems to bring out the enforced opinions in people.
What information would you like to find on the website exactly?
It also has PGP support (with the help of OpenKeyChain to manage certificates) that works great! There are no other well known solutions for applications supporting PGP in Android; this is huge.
People that either want to avoid walled gardens or need proper encryption to their mail need to support this project!