Synonyms that are in the dictionary are marked in green. Synonyms that are not in the dictionary are marked in red.
Antonyms that are in the dictionary are marked in green. Antonyms that are not in the dictionary are marked in red.
It can be any image in an HTML email.
Source: https://www.pcworld.com/article/2161630/how-to-be-tracked-by-email-and-how-to-protect-yourself.html