In Exchange’s default the retention time for a deleted mailbox is set to 30 days. If you have to force purging a mailbox you have to set retention time to 0 days.
Set retention to 0 days:
Set-MailboxDatabase -Identity “MBX-DB001” -MailboxRetention 00.00:00:00
Cleanup Mailbox Database:
Clean-MailboxDatabase -identity “MBX-DB001“
Set retention back to 30 days:
Set-MailboxDatabase -Identity “MBX-DB001” -MailboxRetention 30.00:00:00
Make sure to change the mailbox databases name to your environment.
Cheers,
Chris
Christopher Dargel's Exchange and Windows Blog 