It's unlikely that an attacker uses a well-behaved email-client to attack you. The parent points out that you don't know what your client does if it receives a

It's unlikely that an attacker uses a well-behaved email-client to attack you. The parent points out that you don't know what your client does if it receives a rich-text (HTML,...)-only email and you set your client to render it as plain text. Your client will probably use some sort of parser to tell the formatting from the content and remove the formatting. If that is the "regular" parser, you're still vulnerable.