gmail.com
I guess I've just really missed the point of email clients or something cause people still make them and people still use them.
You might say that is a lot of mail, an extreme case, but when you help people, have clients, customers, sell stuff, etc all communication should be saved in case.
If I had something digital that I really couldn't afford to lose, the first place I would turn is to Google. Same goes for security. They may have their AI bots peruse through my stuff so they can advertise to me but overall I would think data is probably safer in their hands than my own.
I know that's just my opinion. But they don't have a long record of losing data/being hacked.