To resolve this issue, please use the following steps:
1. By using the command “Get-MailboxStatistics -Identity AliasName |fl” obtain the following mailbox settings:
a. Database
b. MailboxGuid
c. ServerName
2. By using the command “Get-MailboxDatabase -Identity “DataBaseName” |fl” obtain the following database settings:
a. Guid
3. Launch the Registry editing tool (“Regedit”) in the Exchange 2010 server that hosting the user mailbox and navigate to:
“HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet\services\MSExchangeIS\%ServerName%”
4. Under the registry key (that was opened in step “3”), find the database GUID (that was obtained in step “2”, e.g. Private-dad6b75a-f6df-42c6-b1c4-8c63c5ef123f”, when the database GUID represent in the suffix behind the “Private-“.
5. Navigate to “QuarantinedMailboxes” registry key:
6. Find the relevant MailboxGuid (that was obtained in step “1”) and delete it.
7. On the Exchange server, restart the service “Microsoft Exchange Information Store”.
MS Jumpstart 