YIL (TIL) I didn't have time to post this yestrday.
I learned that you can expire emails with Protonmain using sieve. You need to use vnd.proton.expire
require ["fileinto", "imap4flags", "vnd.proton.expire"];
then in the script you can use
if address :matches :domain "from" ["*gmail.com"]
expire "day" "3";
so you can discard all the mails automatically in 3 days from your gmail friends
This is a good way to discard like promos you get, because usually they are valid just for few weeks or month maximum.
Then in the message view you have info when the message is expiring