Last time I used K9 was in ~2020 (then I discovered FairEmail) so I'm not married to the old UI in any way, but from the before and after screenshots, this seems like a regression:

1. Firstly, why did this need rebuilding? I know they want to rebrand it from K9 to Thunderbird and I'm happy that the thunderbird team isn't going to be caught up for years trying to rewrite what already exists while K9 gets to benefit from a comparatively very, very well-known name. But what was so fundamentally broken here that it needs to be entirely remade? Could just have applied a color scheme.

2. I had trouble recognising the subject as part of the email (but only in the new layout). I was looking for it and looking for it... and it turns out banner blindness strikes again: it's above where I thought the email started. No problem, that's an issue on day one only, people will get used to that soon enough.

3. The attention grabber on every email is going to be the purple-colored "SHOW REMOTE IMAGES" (caps theirs). I can't recall the last time I've shown remote images. Perhaps a Domino's email where they briefly hid the discount codes in images in 2016/'17? Anyway I hope that can be moved into a menu as it apparently was before.

4. The date has like ten characters available to it. What happens when it's not "10:00 am"? This seems designed by a designer, then the coder comes in and finds that "yesterday" still fits but "2022-07-03 22:47" is going a problem. That means either making design decisions single-handedly and annoying users who were promised (and gave input on) a different design, or going back to the design team and the mailing list for input and on lord no let's just hide the time and get something done today

5. There's suddenly a lot more space on the side. Hope that's configurable, I don't have a large phone to begin with. Probably just me.

6. The text seems bold and blurry. Probably the screenshot just isn't shown at 1:1 size (old layout looks crystal sharp though?) and the bold might be the email in question using a bold tag. One can hope.

7. Recipients hidden when the field has literally ONE word in it ("Alessandro"), there is space for at least another same-sized name and a blank line below. I bet if you click that tiny expand button (good luck not pressing on the user and composing an email to them instead), nothing in the layout has to change to show the three other names. This would annoy the heck out of me if I'd use K9 for work where more than one recipient is common.

8. It says "Thunderbird Ryan" on top. Is that me? Is it reminding me who I am / which account I'm using, and using another two lines of text for that (one for the text, top and bottom each like half a line's worth of borders and spacing)? Sure hope that's default hidden if you have one account only and configure if not.

9. What does a green lock with "OpenPGP" mean? Was the email encrypted or signed? Both? Either? Is transport encryption indicated? Can that also trigger the green lock? I was going to ignore this but was looking for something positive to close out with, scrolled down, noticed the pgp overlay screen and that it's also not mentioned on there, and thought that this is an essential UI element if users are ever going to know what the lock icon guarantees, so UI space needs to be in the mockup if you're drawing what the detail views are going to look like.

10. "Alessandro Castell..." is as far as it gets for the sender in the new layout before cutting it off. On the old layout, "Edgar W. Dijkstra" fit with two thirds of the line still empty. There's something to say for putting more on that line, but not even fitting one name in a field where a name is supposed to go? Interesting choice. Hopefully you know your correspondents by full name already or you'll need to open up another view if you only know them by last name, want to address them as Dear Mr. Castellsomething in a reply...

Some of these are not going to be an issue for most people, but I've tried to look for clearly positive changes and haven't really been able to identify any. Likely most people will run into at least one of these. I wonder still: why in the first place...