What makes you say that? Any e-mail provider can intercept and read any e-mail they want to. This explanation by cock.li is pretty good on this issue:

How can I trust you? You can’t. Cock.li doesn’t read or scan your e-mail content in any way, but it’s possible for any e-mail provider to read your e-mail, so you’ll just have to take our word for it. No “encrypted e-mail” provider is preventing this: even if they encrypt incoming mail before storing it, the provider still receives the e-mail in plaintext first, meaning you’re only protected if you assume no one was reading or copying the e-mail as it came in. When possible, you should use X.509 or GPG with your mail correspondents to encrypt your message content and prevent it from ever being handled in plaintext on our servers. You should also download and delete your mail from our servers regularly, which alone is almost as good as encrypting your mail.

Yes and most vulnerabilities related to the mail service are, I imagine, related to interop requirements of legacy protocol/clients. I haven’t audited their e2ee but I expect it’s on par with other e2ee cloud providers, and IIRC they passed SOC ii.

My distrust pertains mostly to their operations during a future exit scenario/acquisition when users are, presumably, more heavily invested in the various offerings of their extended productivity suite.