A commercial version which I use in production and allows for easy maintenance is LimiLabs Template Engine, been using it for 3+ years and allows me to make changes to the text template without having to update code (disclaimers, links etc..) - it could be as simple as
Contact templateData = ...;
string html = Template
.FromFile("template.txt")
.DataFrom(templateData )
.Render();
Worth taking a look at, like I did; after attempting various answers mentioned here.