The actual SMTP traffic data that is displayed under Statistics > Data Transfer Statistics > SMTP is not correct: the value is 2-3 times more than it should be.
Smarter Mail is used as mail provider.
This is Plesk internal issue with ID #PPPM-1624 which has been fixed in Plesk 11.5 microupdate #46
Apply the latest Plesk microupdates in:
Tools & Settings > Upgrades and Updates > Update Product Components