title: Postal add custom header
body: <p>I am using Postal.NET for email in my C# MVC application:</p>
  
  <p><a href="http://aboutcode.net/postal/" rel="nofollow noreferrer">http://aboutcode.net/postal/</a></p>
  
  <p>I am using SendGrid for sending out emails.
  Sendgrid requires custom header arguments to be sent using the "X-SMTPAPI" header attribute.</p>
  
  <p>How can I add custom header values in Postal.NET? (e.g. myMessage.Headers.Add("X-SMTPAPI", jsonString);</p>