Domains or subscriptions are getting suspended due to mailboxes quota.
Total mailboxes quota limit which is assigned to the subscription is low.
Total mailboxes quota for the subscription or Service Plan.
In addition, set up a corresponding email notification in Tools & Settings > Notifications: check Resource usage limits exceeded by subscription event for that.
According to Plesk documentation:
Total mailboxes quota (Windows hosting) The total amount of disk space in megabytes available for all mailboxes within the subscription.