*We are assuming that you already have an SSL installed on your SmarterMail website site.

Log into SmarterMail as a System Administrator.
Click the Manage icon.
Select the domain where you want the setting enabled.
That domain's general settings will load in the content pane.
On the Security card, enable Force all traffic over HTTPS.
Click Save. When a visitor navigates to your SmarterMail site, their connection will automatically use HTTPS.

NOTE: It's also possible to make this a Domain Default, and then propagate the setting to all domains on the server. This will eliminate the need to do this per domain.