As far as I understand they use a tool called PhotoDNA (AI company acquired by Discord) which they use to scan pictures.

Yeah, like I found the exact same image posted on Twitter since 2 years and it is still up.

I can recommend Jerboa on Android. Used it since the start and never had problems with it (and its lightweight).

Yeah it sucks but at least they have a good Linux AppImage (sadly no BSD). Their Android client is also fine and is on F-Droid.

I am using Tuta and am very happy with it. I like the feature where you can send encrypted Mails to anyone and they’re unlocked via a password (i dont know if others have it). Important for me at least is, that they’re based in germany.