I use dotLiquid for exactly this task.
It takes a template, and fills special identifiers with the content of an anonymous object.
//define template
String templateSource = "<h1>{{Heading}}</h1>Dear {{UserName}},<br/><p>First part of the email body goes here");
Template bodyTemplate = Template.Parse(templateSource); // Parses and compiles the template source
//Create DTO for the renderer
var bodyDto = new {
Heading = "Heading Here",
UserName = userName
};
String bodyText = bodyTemplate.Render(Hash.FromAnonymousObject(bodyDto));
It also works with collections, see some online examples.