If the mailbox is intended to be a standard mailbox (that is, if the mailbox provisioning attribute is set to MBX=ST), or if your mailbox plan type has a maximum size of 5 GB, you can reduce the mailbox size by emptying deleted items, archiving old items, or using other size-reduction methods.
If the mailbox is intended to be an executive mailbox (that is, if the mailbox provisioning attribute is set to MBX=EX), or if your mailbox plan type has a maximum size that is larger than 5 GB, you have to escalate a case to Microsoft to raise the limits on the Mail-User object in the managed environment. To do this, you can submit a support incident online to Microsoft Online Services Support. Or, you can contact Microsoft Online Services Support by telephone .
Note Before you escalate to Microsoft Online Services Support, you should make sure that the provisioning attribute is set correctly. After the migration is complete, the new mailbox limits will be applied according to the mailbox plan type. Your plan type should have a maximum size that is greater than the expected amount of data to be migrated. Otherwise, the mailbox will be unable to send or receive messages.
Article ID: 3052674 - Last Review: 14 Apr 2015 - Revision: 1