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.
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