Mailbox Size Increases when Transitioning to Exchange 2013

If you are transitioning from Exchange 2007/2010 to Exchange 2013, we will see that the reported size of the mailboxes will increase around 30 to 40%! Don’t be alarmed just yet. The actual space used by the mailbox database will not increase by 30% as this only refers to the attribution of space used by each individual mailbox.

Basically, Exchange 2013 now includes in the mailbox size attribute all the properties of items in a mailbox, thus providing a more accurate calculation of space taken by items in a mailbox and, therefore, the whole mailbox itself.

The problem with this is that, during a migration, users might exceed their mailbox quota when their mailbox is moved to Exchange 2013 and be prevented from sending and/or receiving e-mails...

So, in order to prevent this from happening, I suggest increasing everyone’s mailbox quota by 40% prior to their move. You can either do this at the mailbox database level if your users are using the database quotas, or individually on a per-user basis.

For example, if you want to increase by 40% the ProhibitSendQuota value for all users that are not using the database quota defaults, you can use the following cmdlet:

Get-Mailbox -ResultSize Unlimited -Filter {UseDatabaseQuotaDefaults -eq $False} | Where {$_.ProhibitSendQuota -ne "unlimited"} | ForEach {Set-Mailbox $_ -ProhibitSendQuota "$($($_.ProhibitSendQuota).Value.ToMB() * 1.4)MB" -WhatIf}

However, don’t forget to set the IssueWarningQuota or ProhibitSendReceiveQuota if you are using them! Note that this is not the best method as users might end up with a quota of 1523MB for example when the ideal value would be 1500MB...