@Petr

builder.AddCustomHeader(
    "Return-Path", "<wikipedians-owner+bob=example.org@example.net>");

Please note that SMTP server may remove or modify it, replacing it with the value specified in the SMTP ‘MAIL FROM’ command.

You can use build-in Variable Envelope Return Path support, or you can specify ‘MAIL FROM’ address directly:

IMail email = ...

SmtpMail smtpMail = new SmtpMail(
    "wikipedians-owner+bob=example.org@example.net", // MAIL FROM
    new [] { "bob@example.org"}, // RCPT TO
    email.RenderEml()); // DATA

using (Smtp smtp = new Smtp())
{
    smtp.Connect("smtp.example.net");
    smtp.UseBestLogin("user", "pass");
    smtp.SendMessage(smtpMail);
    smtp.Close();
}