The old Opera had an excellent integration with its M2 email client. There's also Seamonkey. Most email programs use a built-in rendering engine anyway, and so

The old Opera had an excellent integration with its M2 email client. There's also Seamonkey. Most email programs use a built-in rendering engine anyway, and sometimes browsers can actually use the email code (e.g. MHTML is basically MIME+HTML, HTML sanitization[0], Opera used M2 for RSS support). So a browser having a built-in email client might be undesirable, but it is not "insane". [0]https://frederik-braun.com/firefox-ui-xss-leading-to-rce.html https://frederik-braun.com/firefox-ui-xss-leading-to-rce.htm...