This subject is very dear to my heart - I had to do so many projects that send emails to customers. One thing I always do is create HTML templates with my own tags inside. I usually have one template for the body and second for the line. This gives me ability to have multiple versions of the same email in different languages for example. Then at run time, driven by data I select appropriate template with a series of simple replace statements plug in data in place of my tags. and send it out.

I remember one project were I had 10 variations of email in 3 different languages, 30 total and was all handled in one procedure.