We have an odd situation, but here it goes:
MailboxOld is a "mailbox" but all email is forwarded to MailboxNew without saving a copy. Essentially, it's just a place to bounce emails off before they get to the final destination (don't ask why ;-) ). We want to completely remove the "mailbox" aspect of MailboxOld BUT leave the email@example.com address in production so it still goes to MailboxNew
Our original thought was to remove the SMTP entry for the address from the old mailbox and add it to the SMTP list for MailboxNew, but with replication, this can cause some downtime for the address, which would be less than ideal.