I know that’s a vague ‘improve’ statement, but it comes specifically from a recent experience emailing someone’s business email, which uses encryption, local storage, and in-house vpn. Not all emails were bounced back, but many were, and others on the thread got their emails to him with no problems. Obviously proton mail is here to provide that level of enterprise security, so making sure it plays well with others that do the same hopefully gets better with time.