Thanks for your answer!
But doesn’t librem.one allow its email users to send and receive unencrypted emails? Do they not need to be stored? Yes, theoretically it is simple to encrypt email e2e and would be quiet complicated to do so in a mastodon direct message. But that is the only difference I can see here. (Upadate: Just found this on the librem.one page: “Safe. We delete unencrypted mail after 30 days.”).
Furthermore I have to emphasize again that I like the thoughts behind the changes purism did to its mastodon instance. That’s why I’d like to discuss this aspect instead of just moving away. I’d like to see these ideas even better 
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I would go this far that even with e2e encrypted DMs it would be nice to have an opt-in for DM and an auto-reply with custom text if the user doesn’t want them. As others argued: DM could also be a way to receive information that is not wanted.
Well, can it be that hard? I’m not a programmer and didn’t even look at mastodons code, but I’d expect there’d be an API for things like DM?