I have the feeling I should know this answer, but have no idea.

Just started using a new emailer, Thunderbird, having used OE for years.

Now, when I compose mail, I am asked whether I want to send in:
1. Plain text only.
2. Plain text and HTML.
3. HTML only.

OE never asked. It just sent the mail.

I have no idea what the differences are. Sometimes I want something in bold or italics, for emphasis, or I want a different font and size.

Which of the three choices should I select?