You can use the Exchange admin center (EAC) or the Exchange Management Shell to change the mailbox storage quotas for certain mailboxes. When a mailbox reaches its specified storage quota, Users receive a notification warning them of the low free space.
When setting custom settings for Mailbox size. There are 3 options to look at:
Issue a warning at (GB): Exchange will send a warning message to the user if their mailbox reaches this size.
Prohibit send at (GB): If the mailbox size reaches or exceeds this limit, Exchange will prevent the user from sending any further messages.
Prohibit send and receive at (GB): If the mailbox size reaches this specified limit, Exchange will prevent the mailbox from sending new messages and won't deliver any new messages to the mailbox.
You can also use the Exchange Management Shell to configure storage quotas for a mailbox.
Set-Mailbox -Identity "Joe Bloggs" -IssueWarningQuota 4.5gb -ProhibitSendQuota 4.75gb -ProhibitSendReceiveQuota 5gb -UseDatabaseQuotaDefaults $false
So, this will set Joe Bloggs’s mailbox limits as:
UseDatabaseQuotaDefaults $false is needed to set custom attributes