Sending an email from OpenCRM sends primarily in HTML but provides a plain text version for non-html email clients.
The plain text version is a stripped down version of the HTML version - with all html formatting and tags removed. The more heavily you design and format the html version of the email, the less successful that plain text conversion will be. For example if you use tables and divs to position content, you may find that the plain text version does not very successfully maintain the order of the content.
If you want to successfully support sending to both HTML and plain text email clients, keep your email design and formatting as simple as possible.