Well, email is already kinda 'fixed'. You can't do much about laggards but they drop out with time. Authentication? DKIM, check. Integrity and confidentiality

Well, email is already kinda 'fixed'. You can't do much about laggards but they drop out with time. Authentication? DKIM, check. Integrity and confidentiality? SMTP-TLS, check. Oh, you meant against the provider? S/MIME, check. Base64 - irrelevant given the size limits most vendors apply to attachments. Email isn't a great file sharing system and trying to optimize it for that might be a bad idea. People use Google Drive etc links these days instead, which works better.