One thing that has been around for several versions of Exchange is the concept of storage quotas. Using quotas, we can control the size of each mailbox to ensure that our mailbox databases don't grow out of control. In addition to setting storage quotas at the database level, we can also configure storage quotas on a per-mailbox basis. In this recipe, we will take a look at how to configure mailbox storage quotas from the Exchange Management Shell.
Use the following command syntax to set custom limits on mailboxes:
Set-Mailbox -Identity testuser ` -IssueWarningQuota 1024mb ` -ProhibitSendQuota 1536mb ` -ProhibitSendReceiveQuota 2048mb ` -UseDatabaseQuotaDefaults $false