- MailEnable is used as a mail server and Horde is used as a webmail for a domain.
- Option Move deleted messages to your Trash mailbox instead of marking them as deleted in the current mailbox? is selected in Horde at Mail > Preferences > Deleting and Moving Messages .
- Search field in the Inbox folder is used to find several messages for deleting and these messages are moved to the Trash folder.
- Then, when selecting all messages in the Trash folder and clicking Delete , only one message is actually deleted, others are moved back to Inbox .
This is Plesk bug with ID #PPPM-6087. The fix is considered to be included in one of the next Plesk updates.
As a workaround, switch domain's webmail to MailEnable WebMail at Domains > example.com > Mail > Mail Settings > Webmail .