Unfortunately this will probably not work. As mentioned here:
>>emails are sent as text. HTML is not supported. <<
However I'm curious. Did you find some workaround for this? My questions if not:
- Can you check the Recevied email format (is it plain text) ?
- Can you try <h1></h1> additionally ?
- What did you get when you sent STRIPHTML("<b>" + " my Text " + "</b>") ? was it only my Text or also the <b></b> ?