That one supposedly has source code available under GPL3 but despite GPL3 clause 9, which says "You are not required to accept this License in order to receive or run a copy of the Program", presents the GPL3 as an EULA and refuses to run unless the user clicks "I agree".
"I agree" is presumably shorthand for "I agree to the linked license", or in other words "I agree to the GPL3", or to expand that, "I agree that I am not required to accept this License in order to receive or run a copy of the Program". Since that's part of the GPL3 text.
I can't be any less of a lawyer, but that sounds OK to me. One is not required to accept the license merely by virtue of it's having been published under that license, but this is surely compatible with the author adding extra conditions to run their program if they so wish?
The copyright holder is not bound by the license granted to others, so in general is legally permitted to add any additional restrictions. However, as this particular additional restriction is not one of the additional restrictions that GPL3 section 7 permits, section 10 applies to anyone other than the copyright holder, which states "You may not impose any further restrictions on the exercise of the rights granted or affirmed under this License." This is a problem for e.g. F-Droid's package, which is neither distributed by the author nor distributed in accordance with the terms of the GPL.
Not by me. The developer is, as far as I am aware, not doing anything illegal, and while F-Droid technically are, the developer as the copyright holder is the only one who could possibly take any action against it and is, I would expect, happier with F-Droid doing what they are doing now than with patching out the EULA acceptance screen.
I didn't know of this one, that looks really nice! Do you know why they chose to start over when K9 already existed for a long time (it says copyright 2018-2021 on the website), or is it a fork?
FairEmail was made from scratch, and has modern (from the ground up) sync logic, that works well with modern battery saving measures in Android. Push mail by default, and low power consumption during sync. I believe it actually works better than Gmail (which has the advantage of Google's push infrastructure) - FairEmail just uses IMAP idle (where possible).
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.
https://f-droid.org/en/packages/eu.faircode.email/