This feature was added in SmarterMail 16.3.6522 (Nov 9, 2017):

To find this option, log in as a Domain Administrator. Click on Domain Settings, then click on Accounts. In the Users grid, select one or more users. Then click on the Actions (...) button and select Expire Password. (Alternatively, you can right-click on the user in the grid to select Expire Password.)

Some notes about this feature: 

If password changes were disabled for the user, this setting will enable password changes and expire the password of the user. In addition, accounts set to Active Directory authentication cannot have their password expired.

When creating a new user account, you can also enable "Force password change at next login".