A few days ago, I had to configure a few mailboxes for enabling message copy while sending. However, this is not the point at this moment. When I ran the PowerShell command, I got the following output:
The error says that this command only works for user or shared mailboxes. In the first moment, this was bit confusing for me. I was sure, that this particular mailbox was a normal user mailbox. However, by checking the recipient type it showed me that I was right with my guess. It was a user mailbox:
The next step was to check the recipient type details, which actually where more important for the issue. The output of this request showed me that this mailbox was still a linked mailbox.
The constellation in this case was, that we had a coexistence between two environments that we were using to migrate from one environment to a new one. The mailboxes and their content were migrated to the new site, but the Active Directory user accounts where still on the old site. In this case the mailbox was in a state that everything was working fine, but I was not able to enable message copy by sending.
Changing the mailbox type
In a best case scenario, we can migrate the Active Directory user to the new site and the linked mailbox would be changed into a user mailbox. However, in my case this was not possible. This is why I went for the hard way.
We go to the Active Directory Users and Computers and navigate to the Attribute Editor. Here, we need to make two changes:
First, we go to the field msExchRecipientType and change the value to 1.
The second thing we need to edit is the msExchMasterAccountSid. This one has to be empty to prevent from running into unexpected troubles.
After changing the entries in the Active Directory, I was finally able to run my PowerShell command for enabling message copy while sending.
By the way: This “hack” works for other mailbox types as well. In the table below, we can see the different values for mailbox types:
Important: Please note that this way should only be followed when there are no other options left. I deeply recommend you to follow the Microsoft best practices to handle linked mailboxes and, if possible, to change the mailbox type the official way. However, this solution is hard-coded and kind of a last resort.