Use the editor to customize the header that is automatically added to your messages: MailUp 8.2 introduced a new HTML editor that uses the familiar "ribbon" interface that you find in MS Office.
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 "Web version" button under the "Insert" tab 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