This issue occurs if a display name isn't specified for the on-premises mail-enabled group.
The following procedure describes how to edit a display name by using ADSI Edit.
- On a domain controller or a computer on which the Windows Server Administration Toolkit is installed, click Start, click Run, type adsiedit.msc in the Open box, and then click OK.
- Right-click ADSI Edit, and then click Connect to.
- Under Connection Point, click Select a well known Naming Context, and then make sure that Default naming context is selected in the drop-down box.
- In the navigation pane on the left side, in the AD DS hierarchy, locate the mail-enabled group that isn't synced to Office 365. Right-click the group, and then click Properties.
- Click Filter, and then clear the Show only attributes that have values option.
- On the Attribute Editor tab, locate the displayName attribute, and then double-click it.
In this example, the value of the displayName attribute is set to <not set>. This is reason why the group isn't synced to Office 365.
- In the Value box, enter a display name for the group, and then click OK.
- Exit ADSI Edit.
- Set the value of the FullSyncNeeded registry entry to 1. To do this, follow these steps:
- Open Registry Editor. To do this, click Start, click Run, type regedit, and then press Enter.
- In Registry Editor, locate the following registry subkey:HKEY_LOCAL_MACHINE\SOFTWARE\Microsoft\MSOLCoExistence
- Right-click the FullSyncNeeded registry entry, and then click Modify.
- Type 1 in the Value data box, and then click OK.
- Exit Registry Editor.
- Force directory synchronization or wait for the Azure Active Directory Sync Tool to sync the on-premises mail-enabled group to Office 365.
For more info about how to install and to use the LDP tool (Ldp.exe), go to the following Microsoft website: