If you receive a message that includes HTML, Yahoo! automatically translate it into styles, sizes, fonts, colors, and so on. Email addresses and URLs are automatically converted into hotlinks in the messages you receive. They will appear underlined and in blue when you read them and be clickable. (If you are reading your email using a POP mail client, the appearance will depend on the email client settings. Please refer to the program's help files for more information.)
If you receive a message in which the HTML tags are visible, it's likely that the sender didn't properly indicate that the message should be treated as HTML. Most email systems, including Yahoo! Business Email, allow senders to choose this option before sending a message. The steps required to do this will depend on the email system.