GPG is a slight improvement that you can use on top of any provider and it will encrypt the content of emails (and content only, metadata will still be there and even subject lines in certain implementations), but both you and the recipient will have to use it and good luck convincing non-IT people to deal with its absolutely atrocious user experience.
In other words, treat it like a physical mailbox. Good enough for various notifications, useless if you're trying to protect anything valuable in it.
For a closed group like a business you can simplify things by having a self hosted email server kept in a secure location. Less secure than end to end encrypted email, but then, few things are as secure as E2EE email. It's hard to beat a medium where the encryption can be done completely offline.
They only allow interaction via the web site or their apps. The apps are nice. But they act more like an IM client.
The biggest issue i had with it, is that they don't support subfolder. So you only habe one level of E-Mail directories.
I moved to mailbox.org now and it seems fine now.
"The former CIA director used a trick often used by terrorists and teenagers to make e-mails harder to trace, the Associated Press reports."
https://www.cnet.com/tech/services-and-software/petraeus-rep...
They probably would be if you were targeted, though, but that's what Tor and VPNs are for.