Newsletter is basically a html page that you send in email. You may recall it getting from different tech giants. Newsletter are usually send each month and contains short information on what happened on that month. It may be on company status, product update and so on. If you insert any html text in Thunderbird the text goes as it is i.e. without being rendered as HTML. So to send newsletter or any html content in email rendered as HTML with CSS support, follow the steps below: First of all make sure that you have enabled 'compose mail in html' option from Edit->Account Settings->Composition and Addressing . Now Go to Compose Mail or Write Mail With body part focused go to Insert->HTML Alternative you can choose other options in Insert menu to accomplish the need.