I uses FastMail after I discover this serious issue about Proton. I would say it work flawlessly and their web mail client is super fast; even faster than Gmail.

Besides, FastMail exists before Gmail and the people in FastMail are active standard protocol developers like IMAP and recently JMAP (a modern mail protocol will replace SMTP/IMAP, FastMail as a reference implementation), which is good because at least I know they understand the protocol and implement it by themselves.