They'd also have to do spam filtering client side, parsing the MIME to extract inline images, attachments and so on, sanitize the HTML to protected against XSS

They'd also have to do spam filtering client side, parsing the MIME to extract inline images, attachments and so on, sanitize the HTML to protected against XSS attacks, process it for full-text search, filtering into labels, auto-forwarding and really absolutely everything that happens to an email. They all involve "reading" the email.