Mailbox usage is always
0Kb under Top > Services > Websites > example.com > Open in Control Panel > Mail for mail accounts deployed to SmarterMail service node.
The following error is shown in
[panel]: Internal error during processing statistic for service node #3[188.8.131.52]:StatisticsCollector capability not supported
Parallels Plesk Automation prior 11.5 MU6 does not support mailbox usage statistics for SmarterMail. SmarterMail mailbox and domain mail usage calculation was implemented in 11.5 MU6, check release notes: http://download1.parallels.com/ppa/11.5/release-notes/parallels-plesk-automation-11.5-updates.html#1156
Update Parallels Plesk Automation to 11.5 MU6 or higher.