Yes, SimplyCast allows you to send messages in both HTML and text format. This is called a multi-part format.
A multi-part message basically is a single email that is sent that contains an HTML version and a Text version. If the recipient can't view HTML, they see the text version instead. The recipient would only see one version or the other, with HTML as the preferred version that would be displayed.
When you create a message (with the exception of choosing text only), you would be taken to the body section of the html content. To create the text version of the message, use the Text tab in the message editor. Text-only content can only contain text, and you cannot format this text with different fonts, font sizes, add links, or insert images.
If you would like to insert links in your text-only email, it is recommended that you enter the links like this: when you include http:// in the path, many email clients that are only allowed to display text only can automatically make those links click-able to the recipient when the message is received.