It has been a while since I posted, but I came across a nuisance error this morning: "This is the home email address of this recipient. It cannot be removed". The address that could not be removed was not the primary address and what's more this was a mailbox, not a contact, so I could not just delete it without disrupting the user. I needed to remove the rogue address as it had been created in error and was a domain that was not managed by Exchange. Anyway, I launched ADSIEdit.msc, found the attribute TargetAddress (http://msdn.microsoft.com/en-us/library/aa487600(EXCHG.65).aspx) and cleared it. I could then go back to ADUC and remove the rogue address. After about 20 minutes mail started to be delivered to the mailbox once again.