1. I use a custom domain.
Turns out that there are two competing features, not-at-all documented. If you use a catch-all, like I do, AND use specific addresses for sending, the two are incompatible to some degree. Which is bonkers.
Example: with a catchall I can create any address I want (and I do). Some store wants an email for a big discount, cool, here's a throwaway. Buying something online, here's a throwaway.
Now sometimes, I need to reply using that throwaway. Turns out in Proton, this triggers a gotcha. As soon as I add the throwaway email to my list of email addresses for sending, I enter a world with a limit of 10 max.
That's fine, I can disable them right?
Nope, it turns out if I disable them in order to add aothers, Proton blocks those addresses *even though I have a catch-all*. WHAT?? Worse, if I try to delete the addresses, Proton will also delete the associated messages in my Inbox/folders. Excuse me?
2. What really pushed me away: Search.
Whatever proton is using under the hood is easily the worst search experience I've ever had from a mail product, and I use Thunderbird on my work machine.
Notable: Proton Bridge. I get why, but it's just terrible.
So many rough edges. Just not worth it.