title: Mailing list replies to go only to the sender, except when reply-all is used
body: <p>I am working on a PHP based mailing list using PHPmailer.</p>
  <p>Currently I have implemented two options for answers to mailing list posts: reply to sender only Vs reply to list. This basically controls which address is inserted in the Reply-To field.</p>
  <p>I want to create the behaviour which my users know from Mailman, here an example:</p>
  <p>SenderA posts a message:</p>
  <blockquote>
  <p>From: senderA@foo.bar</p>
  <p>To: list@foo.bar</p>
  </blockquote>
  <p>The mailing list forwards it to all recipients, e.g. here to RecipientA:</p>
  <blockquote>
  <p>From: senderA@foo.bar</p>
  <p>To: recipientA@foo.bar</p>
  <p>CC: list@foo.bar</p>
  </blockquote>
  <p>Now RecipientA replies to the post and the reply looks like that:</p>
  <blockquote>
  <p>From: recipientA@foo.bar</p>
  <p>To: senderA@foo.bar</p>
  </blockquote>
  <p>The other option RecipientA has is posting the reply to the whole mailing by choosing &quot;reply to all&quot; in the mail client which looks like that in the reply:</p>
  <blockquote>
  <p>From: recipientA@foo.bar</p>
  <p>To: senderA@foo.bar</p>
  <p>CC: list@foo.bar</p>
  </blockquote>
  <p>When I would decide to set the mailing list's address in the CC field for all forwarded mails:</p>
  <ul>
  <li><p>Does that mean the mail comes back to the mailing list 100 times when I send out the mail to 100 subscribers (-&gt; it would cost quite a lot of performance to retrieve, check and drop that duplicate mails)</p>
  </li>
  <li><p>Can I include a certain header so that the mail to the CC'd mailing list is not send 100 times? How can I tell the mail server(s) to not do that?</p>
  </li>
  </ul>
  <p>Or:</p>
  <ul>
  <li>Is there an alternative way to allow the users to decide to reply to the sender or to the whole list?</li>
  </ul>