Hello,
1) Check to see how many emails your clients are currently sending (/etc/virtual/usage) and set the value a bit higher than that (to allow for busier days)... perhaps double would be reasonable. This will vary per system, per user, per day, etc.. so picking a specific number would be up to the Admin and may require ongoing changes to keep a reasonable limit.
2) The row for "Email Deliveries" will be in the bandwidth breakdown pop-up page. When viewing the User stats, there will be a "details" button next to the bandwidth. Clicking this button will bring up the bandwidth breakdown pop-up.
John