I came across the error below while decommissioning Exchange 2010 servers in 2003-2010 co-existence environment (test lab). I was trying to remove the arbitration mailbox using Get-Mailbox –Arbitration | Remove-Mailbox –Arbitration –RemoveLastArbitrationMailboxAllowed and got the error below.
The Arbitration Mailbox “mailbox name” is used by Exchange and can’t be removed.
I can see the arbitration mailboxes using Get-Mailbox –Arbitration. I was able to remove one of the system mailboxes using the command above (The screenshot above only has one system mailbox, the other one being removed).
If you are in a multi domain environment, especially with a blank root domain, you need to run Set-ADServerSettings –ViewEntireForest $true before you can list the arbitration mailboxes using Get-Mailbox –Arbitration.
Though I can use ADSIEdit and remove the AD account, I was thinking of removing the Exchange attributes the proper way, using the Shell. I tried disabling the arbitration mailboxes and it worked (surprise). The command I ran was Get-Mailbox –Arbitration | Disable-Mailbox –Arbitration –DisableLastArbitrationMailboxAllowed
I couldn’t remove the mailbox, but I could disable it. Go figure 
Thank you for this! I was pulling out my (non-existent) hair for almost an hour before I came across this.
Glad it helped you Mike. Let’s save the hair ;-)
disable worked when remove didnt
Thanks for the update Raf
Thank a Lot man !!!!
Glad it helped you Antonio
Thank you. Disable worked where remove did not
Thanks Rob
It works perfectly!!
Perfect! Thank u so much! :)
Glad that the article helped you Rafael.
Thanks.
Thanks! Helped me too with a failed Exchange 2010 server.
Glad the article was of some help Adam.
thx!!!!!!!!
Glad it helped David.
WARNING: The database ‘Mailbox Database 0478292179’ to be accessed on server ‘xxx.xxx.com’ is not mounted or is not available.
Is that the error you get Giorgi?
Can you not mount it manually?