In my previous post I have talked about what happens behind the scenes when you disconnect a mailbox via EMC. Soft deleted mailboxes stay on the system for the whole retention period (by default 30 days) hence they still utilise space within Exchange DB, if you are running on low disk space then this might become an issue.

Luckily there is a solution to this issue, using Remove-StoreMailbox command in Powershell, follow the solution in this MS article http://technet.microsoft.com/en-us/library/gg181092(v=exchg.141).aspx