Before MS, there was a time where it wasn’t possible to get a virus by receiving an email. Users had to click on the attachment. Email itself is a text protocol after all.
Then they added automatic HTML parsing of incoming emails, and the attackers cheered.
Email may be text-based, but parsing MIME is a pain in the ass (speaking as somebody who wrote a console-based email client.)
You can go back to mutt's parsing, and CVE-1999-0940 for an old example. But I'm sure that's only one of many.