Reclaiming Space from deteled mailboxes

Hi,
We run Exchange 5.5, SP4 on NT4, SP6a. After deleting a large number of large mailboxes, i haven’t seen the space recovered by the online defrag (Ie from the results displayed by event 1221). Our deleted items retention time is set to 5 days and this time has well and truely passed.

Do I need to run an offline defrag to recover this space?

Thanks