Workaround: E-Mail from Synchronized Distribution Groups Flagged as Spam
If all of the following conditions exist in your company, e-mail sent from Microsoft Exchange Online to your company’s distribution groups may be filtered as spam and not delivered.
· You have established e-mail coexistence between your on-premises Microsoft Exchange Server environment and Exchange Online.
· You are synchronizing your on-premises Active Directory with Microsoft Online Services.
· This synchronization includes distribution groups that were created in your on-premises e-mail environment.
· Members of these synchronized distribution groups have been migrated to Exchange Online, but still have mailboxes in your on-premises e-mail system.
Why does this happen? Distribution groups have an Active Directory property called ReportToOriginatorEnabled. This property can have a value of True or False. If set to True, the e-mail system receiving the mail will be able to read where the mail is coming from; if False, the receiving e-mail system will not be able to tell where the mail is coming from.
If a distribution group in your on-premises e-mail system has this property set to False, this value will be synchronized with the distribution group in Exchange Online, which in turn means that e-mail sent to members of this distribution group from users with Exchange Online mailboxes will have empty Return Path headers. These empty headers may trigger your spam filtering server or service to filter these e-mails as spam and prevent delivery of these messages to users with mailboxes in your on-premises e-mail system.
Workaround
To ensure delivery of e-mail, set the value of the ReportToOriginatorEnabled property to True for all distribution groups in your on-premises e-mail system. You can do this with LDP.EXE, ADSI Edit, or a script that can modify the ReportToOriginatorEnabled property. After you make this change, this property will be updated in Exchange Online at the next synchronization.
Note: This condition does not affect distribution groups that were created in the Microsoft Online Services Administration Center because these distribution groups are created with the ReportToOriginatorEnabled property set to True.
