I am working on updating the permissions on a few shared mailboxes, however it is failing both through powershell and the GUI. All mailboxes, both user and shared are in O365 and have been for over a year. When trying to add "send as" permissions for a user to a shared mailbox, PS and the portal error stating that the domain is not accepted in the organization. This behavoir occurs for any existing shared mailbox, with any of the 3 domains that are setup in O365, there are no mailflow issues to any of the mailboxes. Creating a new shared mailbox, I can add and remove permissions without any issue.
Any thoughts on what may be affecting the permissions on existing shared mailboxes?