Disabling a mailbox because its domain needs a rest is sensible — unless three live campaigns were sending through it, in which case you have just stalled them, and you will find out from the numbers next week.
The mailbox list now has an Active Campaigns column, and opening a mailbox shows exactly which campaigns depend on it. The consequence of turning it off is visible before you turn it off.
Assigning mailboxes to a campaign also got a searchable typeahead, which matters once you run more mailboxes than fit in a dropdown.