If you wrote or created an email message in another application (such as Adobe Dreamweaver), you can upload your HTML document into MailUp through the Create new email from file feature.
Type the subject line (i.e. title of email) under the Subject field, write any notes for your reference, and browse to select the file to upload. Click on Save. Once the message is saved, you can further work on it and make any final changes (click on Modify) through the built-in HTML editor.
Creating a new email from URL is used to retrieve the content (i.e. body text) of a webpage into your email message.