Use the editor to customize the header that is automatically added to your messages. The editor resembles that of a common word processor; the question mark button 
will help you get the meaning of every button.
Make sure your email messages always contain a link to a web version of the message. This will avoid a situation in which recipients cannot read the message due to display issues. Use the button 
to insert the link. The header already contains a default version of the link.
If you want to have a complete overview of how the built-in editor works, go to this page
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