There's not a feature in SmarterMail itself that will cap bandwidth usage at X amount per month. What you CAN do is set up throttling on outgoing bandwidth that will at least limit the amount used on a per hour basis. You also have the ability to delay or reject mail once the domain reaches that throttling limit. If you were to set up bandwidth throttling, you can calculate a domain's usage on a monthly basis if they were running at the throttling limit daily throughout the month and go from there. If you're using a control panel for creating your hosting plans, that may have an option for bandwidth calculation and limits as well.
What we did at the hosting company I worked for was use our firewall and border routers to measure usage, then bill overages if a specific IP went over our allowed allocation. Using your networking equipment is a bit more efficient versus having the software calculate usage for billing purposes as it should give you more options for what to do once a limit is reached. Of course, we had a single bandwidth allocation for ALL Web-based activity: mail, website visits, etc. and didn't count internal usage (e.g., communication between a website and a separate SQL server) against the monthly allocations.
I hope this helps. Maybe others have some input.