I use ProtonMail for my (ahem!) "dirty work," but that's a very small volume of e-mail. Sometimes I go two months or more without sending or receiving a message at that address. If I had the volume of mail in ProtonMail that I get on my "real person" account, I'd have a hard time keeping up with all the deletions, junk and spam.

My solution of "don't receive or send a lot of e-mail" works for me, but I can see where it wouldn't work as a business account. I don't know if the paid version gives you the flexibility of multiple folders and self-sorting. I was disappointed that it (the free version) didn't have that feature, but like I say, I don't have enough traffic to warrant the conveniences of my regular mail client.

By the way, I use Thunderbird for my regular e-mail and it works great when it works, but it crashes every few days. As far as I know it hasn't lost any of my mail, but I can't be sure.