Now you can see POP/SMTP traffic statistic for domain and there is no possibility for mails counting for mailbox. This process require a lot of resources and can lead to overloading on system where a lot of mails should be sent/received.
You can try to use some third-party software for this purpose, Qmail-Scanner Statistics, for example. But it was not tested and you can try it on your risk. Any your experience would be very interesting for this Plesk community