unfortunately what Shahed said is completely wrong!
you know! in emails we have something called Header, just like the header in HTML, which define some meta data like MIME TYPE or Encoding. if you set your MIME Type to plain text, then even though you put some HTML code in your email, it will be shown just like normal texts and the HTML codes are ignored (you can see the code!) it's just because of the header that is set to plain text. you claimed that you want to speak in plain text language not HTML!
for some security reasons MT will only send plain text emails. it means that in header the MIME Type will be automatically set to plain text. if you want to change it is needed to hack the MT code. it would be helpful to take a look at this file MT_DIR/lib/MT/App/Notifylist.pm if you are familiar with perl and MT Objects.