- Plesk Onyx for Windows
- Subscription has been migrated to the new server including mail accounts. After migration already read messages for mail accounts recognized as new by mail client and received again.
- MailEnable is installed on the source and the target server.
- Mail client (Outlook, Thunderbird, etc) is configured to use POP3 protocol.
This is Plesk bug with ID #PMT-4208 that is planned to be fixed in future Plesk updates.
As a workaround, use IMAP instead of POP3 as a protocol for the mail account while configuring mail client.
Information how to configure mail client can be found in documentation: