My technique is to have a BO (mine's called StandardText) with a text attribute set to html format (mine's called Content).
I then have a notification that has <<StandardText.Content>> in the message area (no need to set to html format - just leave it as plain vanilla text)
My process does a pick from so I can select which standard text to use, then does the SEND
I tend to use Dreamweaver to design the content for the email and then just paste the code into StandardText.Content records - first clicking the "source code" button in the AwareIM html editor.
I wish I could do it all within AwareIM but their html editor is rubbish. One wonders why they haven't given us FCK by now but hey-ho, that's life. But if you are a subscriber to Adobe Creative Cloud suite - it's probably the best route for designing swish mail shots anyway
Hope this helps