Hi our, I'm trying to use a Mandrill subaccount when sending emails from our Umbraco Cloud site. The main account is set up correctly in web.config like this:

 <system.net>
    <mailSettings>
      <smtp>
        <network host="smtp.mandrillapp.com" port="587" userName="{USERNAME}" password="{PASSWORD}" />
      </smtp>
    </mailSettings>
  </system.net>

To use the subaccount, I need to add a custom header to all messages sent from the site. After some seaching around, I can see that it has to be added to the MailMessage like this message.Headers.Add("X-MC-Subaccount", "{SUBACCOUNT-NAME}");.

How can I achieve this, when I don't have access to the code where the mail is sent? I presume it's hidden in the Umbraco.Forms dlls.

Thanks!