This is actually a very tricky problem. We solve this by never rendering mails in trusted origin. It's basically sanitisation + isolated sandboxed iframe for defence in depth + standard security stuff like CSP headers. When I was looking into this, I was surprised how many popular mail clients relay primarily on sanitization and render mails within trusted origin (ex: marco, superhuman).

Other interesting problem to solve for mail clients is the editor. You should be able to forward/edit untrusted html content. Source: I develop tecotype.com for mac/linux/windows.