You can copy and paste content from another source, but keep in mind that the formatting of your message can change greatly because the code engrained in that specific text (for example, if it comes from another web page, Word document, etc) comes along with the text when it is copied and pasted. It affects the HTML and how your message looks and interacts with different email clients (Gmail, Yahoo!, etc.).
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In other words, you may be copying and pasting more than just the text that you see. You may be adding some essentially hidden HTML coding that can affect the overall formatting of your message.
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You can edit out the extra HTML affecting the message (font, font size, style, etc.) by going to the text editor of your message. To do this, click on the message you are trying to edit from the Automated Messages page and scroll down to the text editor. From here, you either adjust your formatting manually by using the editor's formatting buttons, or you can completely remove the formatting. To clear your message's formatting, highlight the content you wish to clear the formatting for, and from the top of the text editor, click on the eraser icon (Clear Formatting button). Clicking this will revert all the text back to being of the same format.

You could then click on the Save & Send Test button to receive a test message and verify everything is looking as it should.