Hi, we're receiving the below error when trying to add SMTP addresses to Office365 mailboxes from Adaxes. I've raised a ticket with support, but thought I'd ask here as well. 
I've no idea when this started happening, but we could still create new mailboxes last week as I personally created one. 
Error: 
An Azure Active Directory call was made to keep object in sync between Azure Active Directory and Exchange Online. However, it failed. Detailed error message:
Unable to update the specified properties for on-premises mastered Directory Sync objects or objects currently undergoing migration.
The issue may be transient and please retry a couple of minutes later. If issue persists, please see exception members for more information.
Any help would be great. 
Thanks
Gary