Hi Stephen,

I hope all is well. The best way around this is to close the mailbox. This can be done by going to the Email and Conferences Module > Settings > Mailbox Browse, from here you can open the mailboxes that you are no longer use and mark them as 'Closed'.

If you are still struggling you can change these mailboxes to conferences. In the same location as the above, open the Mailbox you wish to change, change the 'Class' to Conference and remove the User Signature before saving.

Many Thanks,
Jack
IT System Integrators