[koakuma-chan]

You can just send a link, and the user's client will probably highlight it even if it is plain text.

[recursivegirth]

Yea, but how will they hide all the tracking URLs and base64 encoded PII from you in the email?

[koakuma-chan]

Using a URL shortener obviously. But you are right, if they only send plain text, they won't be able to include those 1x1 images at the bottom to track whether you have opened the email. Any sane email client blocks images by default, but whatever.