How to limit message throughput per mailbox in Mailenable?
There is no option in Plesk to limit the number of mails sending by a mailbox. This feature is not supported in MailEnable Standard and Professional versions as well.
If you need to avail this feature, you need to upgrade your mail server to MailEnable Enterprise or Premium. However even after upgrade, you need to set the limits through MailEnable Administrative Console only.
Please follow link to see the feature comparison of MailEnable product editions:
Restrict message throughput per mailbox
Restrict message throughput per post office