Gmail on Android complains when a mail server cert changed (good) but doesn't let you proceed to say it's ok (bad).
Let's Encrypt's certs are valid for 3 months.
EVERY THREE MONTHS I HAVE TO REMOVE AND RE-ADD MY MAIL ACCOUNTS
...thanks, crappy Gmail app -_-
@Tungro I'm really sorry this stems from my doing. Quite honestly, I'd done it to /avoid/ cert change issues, and you're the first to report any problems. Do let me know if you need me to put it back.
Alternatively, if you're considering a new client that I'd consider better (I know this is personal opinion), do take a look at K-@: https://play.google.com/store/apps/details?id=com.onegravity.k10.free&hl=en_GB