You can use Sender property of the MailMessage class to set the ReversePath as aspnetmail. for example:
MailMessage msg = new MailMessage(); msg.Sender = "sender@gmail.com"; msg.From = from@gmail.com"; msg.To = recipient@gmail.com; Moreover, the MailMessage also provides more support for email header. You can feel free to add any custom mail header like
MailMessage msg = new MailMessage(); msg.From = "from@gmail.com"; msg.To = "recipeint@gmail.com"; msg.Headers["return-path"] = new MimeHeader("return-path", sender@gmail.com);