title: Sending email with a custom envelope sender (VERP)
body: <p>I'm writing a server that will send email to many recipients on behalf of my client.</p>
  
  <p>The email must come from the client's email address (client@example.org), but I want to automatically handle bounces via <a href="http://en.wikipedia.org/wiki/Variable_envelope_return_path" rel="nofollow">VERP</a>.  Basically, the email <code>From:</code> header will be client@example.org, but the SMTP envelope sender (<code>MAIL FROM</code>) will be <em>unique-email-id</em>@my-email-service.example.com.</p>
  
  <p>I've already built a multi-threaded sending engine that uses the built-in <code>System.Net.Mail.SmtpClient</code> to actually speak SMTP with the recipient's MX server.</p>
  
  <p>Unfortunately, <code>SmtpClient</code> does not allow you to specify the envelope sender &ndash; it just uses the <code>From:</code> address.</p>
  
  <p>I need an alternative that allows me to specify the SMTP <code>MAIL FROM</code>.  Preferably, something that takes little work to drop in and replace <code>SmtpClient</code>.</p>
  
  <p>Thus far, everything I've looked at is an entire <a href="http://mailsystem.codeplex.com/" rel="nofollow">email</a> <a href="http://www.lumisoft.ee/lsWWW/ENG/Products/Mail_Server/mail_index_eng.aspx?type=info" rel="nofollow">suite</a> (SMTP/POP3/IMAP/kitchen sink).  What lightweight SMTP libraries are available?</p>