I just figured out that the text fragment is only invisible when the original message is plain text only.  All is well when there is an html version.

<p>Hi All,</p><p>I have added a rule in Mercury that adds a text fragment an have since been made aware that it is getting added (visible in Message Source) but is not visible in the body of the message by both Thunderbird and on iPhones.  I'm guessing it is because it isn't in HTML format.  Below is the body of a test I sent from a Gmail web interface.  I would appreciate help with what the HTML code needs to look like that should be added to the text fragment file. </p><p>--00000000000018245605a2b3581a Content-Type: text/plain; charset="UTF-8" test of text fragment with ending carriage return CAUTION: This email originated from outside of the company! Do not click links, open attachments or reply, unless you recognize the sender's email address and know the content is safe! --00000000000018245605a2b3581a Content-Type: text/html; charset="UTF-8" <div dir="ltr">test of text fragment with ending carriage return</div> --00000000000018245605a2b3581a--  </p>