Hello Nathan,
I wanted to give you a heads up that the ability to force all traffic over HTTPS will be a native setting option in SmarterMail 16.x, which is currently in BETA and scheduled for final release this first quarter of the year. Going forward, once your site is configured in IIS and a valid SSL cert is installed on the server, you can enable a setting in SmarterMail to require an SSL connection. This setting will be found in the Miscellaneous Settings > Server Info section. The Administrator will see the following settings: Require SSL and Allow domains to override SSL Requirement.
For versions earlier than 16.x, we would recommend following Scarab's suggestion of adding your URL rewrite on the server level instead.
Thank you!
Andrea Free
SmarterTools Inc.
877-357-6278
www.smartertools.com