A forum I belong to requires that messages be sent in plain text - with no encoding or attachments.  I have the font set to courier new, have no attachments and have typed URL's in so that they are not active links.  I'm still told that forum administrator that my message was sent:

"in a non-plain-text format.  The most likely cause is that your mail program appended an encoded (HTML - web page formatting) version of your message."

How can I send a message from Outlook.com using a plain text format only?