Plain text part
The text-only version carried alongside the HTML one inside the same message.
A properly formed email is sent as two versions in one message: the HTML one and a plain text alternative. The client picks whichever it can display, and some readers set theirs to prefer text.
Sending platforms will generate the text part automatically, and it usually shows: stripped markup, naked URLs, no structure. Writing it takes a minute and reads like something a person meant to send.
It is also a mild deliverability signal — HTML-only messages have long been associated with bulk sending — but the honest reason to include it is the people who read it.