I know of no way to do this in SmarterMail itself.

If you have the IIS URL Rewrite Module installed on your SmarterMail Server I would just create a Rewrite Rule to take any url string for unsubscribes from the Hostname FQDN to the Webmail FQDN and another Rewrite Rule to redirect HTTP to HTTPS.

 

Some sample rules to do this would be as follows:

 

            <rewrite>
              <rules>
                <clear />

                <rule name="Hostname to Webmail" stopProcessing="true">
                    <match url="(.*)" />
                    <conditions logicalGrouping="MatchAll" trackAllCaptures="false">
                        <add input="{HTTP_HOST}" pattern="^mx1\.tld\.com$" negate="true" />
                    </conditions>
                    <action type="Redirect" url="http://mail.tld.com/{R:1}" />
                </rule>

                <rule name="Smartermail SSL" stopProcessing="true">
                    <match url="(.*)" />
                    <conditions logicalGrouping="MatchAll" trackAllCaptures="false">
                        <add input="{HTTPS}" pattern="Off" />
                    </conditions>
                    <action type="Redirect" url="https://mail.tld.com/{R:1}" />

                </rule>
            </rules>
        </rewrite>

 

The only problem to this method is that SmarterMail overwrites the \MRS\web.config every single time you install an update, meaning you would have to COPY+PASTE the rules back in to your \MRS\web.config post-update.