Send HTML formatted email

Question

I have the following function that generates the body of an email based on an HTML file:

private string createEmailBody(string userName, string title, string message)  
{  
    string body = string.Empty;  
    //using streamreader for reading my htmltemplate   

    using(StreamReader reader = new StreamReader(Server.MapPath("~/HtmlTemplate.html")))  
    {  
        body = reader.ReadToEnd();  
    }  
    body = body.Replace("{UserName}", userName); //replacing the required things  
    body = body.Replace("{Title}", title);  
    body = body.Replace("{message}", message);  
    return body;  
} 

This function was written a few years ago. Is there any newer way to send HTML formatted email? (I researched in SO but found only old questions from 5 years ago and more.)

I'm using this code in my Web API to send confirmation emails to new registers.