PLEASE BE ADVISED THAT IT IS REQUIRED TO HAVE URL REWRITE INSTALLED IN IIS IN ORDER FOR THIS TO WORK PROPERLY. A HUGE THANK YOU TO ALI MUALLA FOR THIS ADDITIONAL INFO!

The following will redirect ONE domain from http to https in SmarterMail 15x (may work for 16x and 17x, but was not tested on those versions). Add the following to your SmarterMail web.config file in the <system.webServer> area...

<rewrite>
  <rules>
    <rule name="Imported Rule 1" stopProcessing="true">
      <match url="^(.*)$" ignoreCase="false" />
      <conditions>
        <add input="{HTTP_HOST}" pattern="^mail.exampledomain\.com" />
        <add input="{SERVER_PORT}" pattern="80" ignoreCase="false" />
      </conditions>
      <action type="Redirect" redirectType="Found" url="https://mail.exampledomain.com/{R:1}" />
    </rule>
  </rules>
</rewrite>