A former employee's account is disabled in Active Directory and the mailbox has been converted to a shared mailbox. I'm trying to configure an automatic reply through the Exchange admin center and Exchange Online PowerShell, but both return "Object reference not set to an instance of an object." The mailbox folders are well below quota, and the account still exists in Active Directory and Entra ID. Has anyone run into this or found a reliable way to configure the automatic reply?
4 Answers
Check whether there are duplicate accounts or conflicting mailNickname values. An identity collision can cause this sort of vague error. You may be able to configure the reply through PowerShell by targeting a different unique identity property instead of the display name or primary address.
If the response only needs to be sent to particular incoming messages, a mail-flow rule can provide an alternative. It may be easier to manage than an out-of-office setting, but make sure the conditions and exclusions prevent it from replying repeatedly or to internal system messages.
Try temporarily granting yourself access to the shared mailbox, wait for permissions to apply, then open it directly in Outlook on the web and configure the automatic reply manually. Remove your access afterward. This is often quicker than troubleshooting the admin interface.
You can also test converting the shared mailbox back to a regular user mailbox and configuring the reply there, although the same error may indicate the problem is with the mailbox identity rather than its type.
I tried converting it back to a user mailbox, but the automatic-reply setting still failed with the same error. Re-enabling the account and signing in as the user is possible, but I was hoping to avoid that process.