Seemingly text-only email containing blocked content

I received an 'offer' email today in my yahoo account which seemed to contain only a small amount of text.

When the email was received in my email client however (Thunderbird) it had some 'remote content' blocked.
Note that the text contained no links, no URLs of any kind, no images etc.

Sender used Gmail (naturally)

Can anyone explain how can a strictly plain text email contain something that can be blocked?

thanks