How to fix error "Not a valid mailbox"

Created by Abderrahim Ibnou el kadi, Modified on Wed, 22 Jan 2020 at 07:31 AM by Abderrahim Ibnou el kadi

Sometimes the configuration between Proofpoint Essentials and the active directory or Office 365 gets a bit off and, since we don’t have access to AD/O365, we don’t understand why some users get deleted after the synch happened preventing them from getting mail and most of them get a rejection error:

The user Is not a valid mailbox.


To solve this issue, we will add the user(s) to the Proofpoint Essentials portal then go to Company Settings> Import Users.

Active Directory

If the user is on active directory then click on Search now button. The user just created will appear under the Deleting section, then all you need is to click on the Exempt From Sync and the user will appear under the Exempt From Sync section. this user will never get removed from this list of users.



Azure Active Directory

If the user is on an O365 then click on Azure Active Directory, then Search now button, in the display you will get that there is one item to be deleted. Note that this item is the user you just created.


Expend the Deleting section and to the right side click on Exempt From Sync button so the user does get excluded the next time there is a Sync. 


The user will now be listed under the Exempt from sync section, expend the Exempt From sync to validate that the user has been added to the list



If you are using a CSV method to populate your user list, then you don’t need further steps to make since there is no Sync occurring.