How can I detect that mailbox/domain or subscription are blocked by Outgoing Mail Control because of limits?
When the message rate limit set in Plesk is reached an email notification is sent to the server administrator. The rates are checked and notifications are sent periodically with the default period of one hour. Non-admin Plesk users can check if their mailboxes are locked because of the limits were exceeded in Plesk GUI on Websites & Domains > Outgoing Mail Control
To change the period of notifications about attempts to exceed limits on outgoing mail please follow Plesk guide