Adding Custom Attributes to an Office 365 User Mailbox
Microsoft Office 365 mailboxes include 15 extension attributes that you can use add information about a recipient, such as an employee ID, organizational unit (OU), or some other custom value for which there isn’t an existing attribute.
To add or modify data in these Attributes you first need to log into the Office 365 portal as an administrator.
You then need to navigate to the Exchange Administration Console and select Recipients.
Double Click on the mailbox that you wish to add or modify the custom attributes for and click the More Options… link to show the custom attributes.
You can now add or modify these attributes.
If you are a Cogmotive Office 365 Reports customer you will be able to use these attributes to further filter and search your Office 365 reports.