Update 7/10/17: Added a line about OU consideration to Important Notes section.
Note: this solution is for hybrid deployment, where public folders are active in on-premises environment and emails are accepted in Exchange Online. The solution requires both of the following:
On-premises Mail Enabled Public Folder (MEPF) objects synchronized to AAD using AAD connect
Use mail public folder script to synchronize on-premises MEPFs to Exchange Online.
Also, the sync to AAD using AAD connect must be turned off when you are migrating public folders to Exchange online.
Until now, our on-premises customers who use Mail Enabled Public Folders (MEPF) could not use services like Directory Based Edge Blocking (DBEB). If DBEB is enabled, any mails sent to Mail Enabled Public Folders (MEPF) will be dropped at the service network perimeter. This is because, DBEB queries Azure Active Directory (AAD) to find out if a given mail address is valid or not. Because Mail Enabled Public Folders (MEPF) are not synced to Azure Active Directory, all MEPF address are considered as invalid by DBEB. Sender of the mail to MEPF would receive following NDR:
To resolve this issue, in the latest Azure AD Connect tool update, we are introducing an option to synchronize MEPFs from on-premises AD to AAD. Admins can do this through the newly introduced option - 'Exchange Mail Public Folders' in Optional Features page of Custom installation during Azure AD Connect tool installation/upgrade. When you select this option, and performs a full sync, all the Mail Enabled Public Folders from on-prem AD(s) will be synced to AAD. Once synced, you can enable DBEB. Mail Enabled Public Folders addresses will no longer considered invalid addresses by DBEB. And messages will be delivered to them like they are delivered to any other recipient.
Directory Based Edge Blocking is not yet supported for Mail Enabled Public Folders hosted in Exchange Online. Current feature enables DBEB support only for Mail Enabled Public Folders hosted On-premises.
For Exchange Online Protection (EOP) Standalone i.e., customers who have only Exchange on-premises configured but no presence in Exchange Online, and no “advanced” features of EOP, this synchronization through AAD Connect tool is enough for DBEB to work.
For Exchange Online (ExO) & EOP i.e., customers who have both on-premises Exchange & Exchange Online configured, or who are using features such as DLP or ATP, this feature does not create the actual public folder objects in the Exchange Online directory. Additional synchronization via PowerShell is required for DBEB to work if you are using Exchange Online.
For customers who are planning to migrate Public Folders from on-premises to Exchange Online: nothing in the migration procedure has changed with this feature support. One extra point you should take care of before starting Public Folder migration to EXO is – ensure ‘Exchange Mail Public Folder’ option in Azure AD Connect tool is *not* checked. If it is checked, uncheck it before you start migration. By default, it will be unchecked.
In order to perform Mail Enabled Public Folders sync, along with checking 'Exchange Mail Public Folders' feature in optional features page, we need to ensure ‘Microsoft Exchange System Object’ OU of every forest from which we want to perform Mail Enabled Public Folders sync should be checked. This is present in ‘Domain and OU filtering’ page. By default, this option will be checked. NOTE: If OU was unchecked previously & it is being checked again, then full-sync has to be performed. Whenever there is any change of OU options, we need to do a full-sync for the changes to get reflected.
Customers who had a work-around in place
There were some customers who did not want to disable DBEB despite having Mail Enabled Public Folders. These customers have opted for a work-around of creating MSOL objects (like EOPMailUser, MailUser or MailContact) in Azure Active Directory with same SMTP addresses as Mail Enabled Public Folders so that these addresses are considered as valid addresses by DBEB. Customers who opted for this work-around are requested to remove all such MSOL objects before performing the sync of Mail Enabled Public Folders through AAD Connect tool. If the ‘impersonation objects’ have not been removed prior to the new synchronization, they are likely to cause a soft-match error. In soft-match error case, sync of Mail Enabled Public Folder from on-prem AD to Azure Active Directory will not succeed, and an email similar to the following will be received:
"Identity synchronization Error Report: <Date>"Unable to update this object because the following attributes associated with this object have values that may already be associated with another object in your local directory services: [ProxyAddresses SMTP:SampleMEPF@mail.contoso.com,smtp:SampleMEPF@contoso.com;Mail SampleMail@mail.contoso.com;]. Correct or remove the duplicate values in your local directory. Please refer to http://support.microsoft.com/kb/2647098 for more information on identifying objects with duplicate attribute values.
As mentioned in the description, you can correct or remove the entries with duplicate SMTP address. Below are corresponding links for each scenario:
Once the objects have been cleaned up, performing a full sync will ensure Mail Enabled Public Folders are successfully synced to Azure Active Directory. More info here: http://support.microsoft.com/kb/2647098.