Empty inbox is shown in webmail on a Plesk server: Cached message size larger than expected

Symptoms

• When Dovecot is utilized as the IMAP/POP server, an empty inbox is displayed in Horde/Roundcube webmail and email clients like Thunderbird or Outlook.

• The notification icon displaying the number of emails in the Inbox is visible in both webmail and external mail clients, but the actual emails within the Inbox are not visible.

• The following error is found in /var/log/maillog:

  dovecot Error: service=imap, user=username@example.com, ip=[127.0.0.1]. Mailbox INBOX: UID=33261: read(/var/qmail/mailnames/example.com/username/Maildir/cur/1520269963.M402118P28216V0000000000000802I00000000001840FA.CL0016UC-SG1.EXAMPLE.COM,S=1448:2,S) failed: Cached message size larger than expected (1448 > 1188, box=INBOX, UID=33261)
  dovecot: Error: service=imap, user=username@example.com, ip=[127.0.0.1]. Corrupted record in index cache file /var/qmail/mailnames/example.com/username/Maildir/dovecot.index.cache: UID 33261: Broken physical size in mailbox INBOX

• The following error is shown on attempt to enter Roundcube > Sent folder:

  UID SORT: external error occurred. Refer to server log for more information. [2020-01-22 20:05:18] (0.218 + 0.000 + 0.217 secs).

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  IMAP error reported by server

• Some emails may be shown with "Invalid Address" for Sender and "[No Subject]" for Subject:

  

Cause

Mailbox cache file mentioned in the error is broken or corrupted.

Resolution

1. Connect to the server via SSH.