Enabling HTML format

You can change the format of an email message from plain text to HTML in the email composer by choosing Format ▸ HTML from the menu bar.

Alternately, you also can change the first dropdown list below the Subject line from Plain Text to HTML.

If you choose HTML format, a second tool bar will be displayed below the Subject line with HTML-only options.

To send all your mail as HTML by default, enable Edit ▸ Preferences ▸ Composer Preferences ▸ Default Behavior ▸ Format messages in HTML.