Resolution

Ability to use "Sent Items" and "Deleted" folders is possible only with IMAP service. By default Plesk uses MailEnable standard edition as a primary mail server, which does not support IMAP protocol.

To be able to use folders, including the "Sent Items" and "Deleted",  a mail server that supports IMAP service should be used.

IMAP service is supported by the following mail servers that can be used with Plesk:

1. MailEnable Professional Edition.
2. MailEnable Enterprise Edition.
3. Merak Mail Server.
4. SmarterMail mail server.

Additional information

Please see also Horde webmail shows all new messages as read