<p>When you send an email using C# and the System.Net.Mail namespace, you can set the "From" and "Sender" properties on the MailMessage object, but neither of these allows you to make the MAIL FROM and the from address that goes into the DATA section different from each other.  MAIL FROM gets set to the "From" property value, and if you set "Sender" it only adds another header field in the DATA section.  This results in "From X@Y.COM on behalf of A@B.COM", which is not what you want.  Am I missing something?</p>

<p>The use case is controlling the NDR destination for newsletters, etc., that are sent on behalf of someone else.</p>

<p>I am currently using <a href="http://www.aspnetemail.com/" rel="noreferrer">aspNetEmail</a> instead of System.Net.Mail, since it allows me to do this properly (like most other SMTP libraries).  With aspNetEmail, this is accomplished using the EmailMessage.ReversePath property.</p>

Do you mean this?:
```
```
//create the mail message
 MailMessage mail = new MailMessage();

 //set the addresses
 mail.From = new MailAddress("me@mycompany.com");
 mail.To.Add("you@yourcompany.com");

 //set the content
 mail.Subject = "This is an email";
 mail.Body = "this is a sample body with html in it. <b>This is bold</b> <font color=#336699>This is blue</font>";
 mail.IsBodyHtml = true;

 //send the message
 SmtpClient smtp = new SmtpClient("127.0.0.1");
 smtp.Send(mail);
```
```
From
http://www.systemnetmail.com/faq/3.1.2.aspx