I use ProtonMail with providers. Beware; today it was flaky. I sent an email confirming an appointment and was concerned when I didn’t get a reply. I found out later that she did reply but I never received it. As a test I sent an email to my regular email account; it never came.

I switched back to my Gmail..

Happened to me a while ago.  Unless you pay for an upgraded account you have very limited storage.  You probably hit your limit and need to delete emails from your inbox and trash.

and it worked fine. Perhaps the problem is sporadic or has been resolved.

with lag on sending and receiving,  and it has also sent priority messages to my spam folder a few times.

I have recently been receiving spam from Protonmail providers that I do not know.

I emptied the trash and upgraded to the “plus” version that gives me 10x the storage. Now all is well.