When I copy and paste some text, a window opens saying "Do you want to allow this webpage to access your clipboard?". Why?

The message (shown below) is due to an Internet Explorer security setting, as the browser does not recognize MailUp's control panel as a trusted website. To add the platform's address to the list of trusted sites, go to "Tools > Internet Options > Security > Trusted Sites", click on "Sites" and add the URL of your MailUp platform.