Unable to receive emails in Plesk even with enough space available: 4.2.2 Mailbox full

Symptoms

• Emails are not reaching inbox.

• The following error messages show up in /var/log/maillog:

  postfix/cleanup[31600]: 7EC5782366: milter-reject: END-OF-MESSAGE from srv.example.net[203.0.113.2]: 4.2.2 Mailbox full; from=user@example.net to=jdoe@example.com proto=ESMTP helo=<srv.example.net>

• Plesk uses Web Admin Edition License
• Mailboxes have either enough space or set to Unlimited while checking in Mail section in Plesk:
  

• Mailbox quota is set in Service plan and in Plesk database:

  # plesk bin service_plan --info Unlimited | grep 'Mailbox size'
  Mailbox size 1 GB

  # plesk db "SELECT d.name AS domain_name, s.id AS subscription_id, l.value AS mbox_quota_bytes, ROUND(l.value / 1024 / 1024, 2) AS mbox_quota_mb FROM Limits l JOIN Subscriptions s ON s.id = l.id JOIN domains d ON d.id = s.object_id WHERE l.limit_name = 'mbox_quota'"
  +--------------------------------------------+-----------------+------------------+---------------+
  | domain_name | subscription_id | mbox_quota_bytes | mbox_quota_mb |
  +--------------------------------------------+-----------------+------------------+---------------+
  | example.com | 1 | 1073741824 | 1024.00 |
  | example.org | 2 | 1073741824 | 1024.00 |
  +------------------------------+-----------------+------------------+---------------+

Cause

Domain was migrated from another Plesk server with High license type like Web Host Edition, where mbox_quota can be configured.
In Web Admin Edition, the Mailbox quota must be Unlimited.

Resolution

1. Connect to the server via SSH

2. Set Mailbox quota to Unlimited with the following command:

   # plesk bin subscription_settings --update example.com -mbox_quota -1
   SUCCESS: Update of domain 'example.com' completed.