After user's mailbox is switched mail flows directly to the target mailbox and nothing is saved to the source mailbox. In some situations there is a need to switch mailbox back to the source environment. At this moment source mailbox only contains mail up to the moment when the switch to the target occurred. What happens to the new mail that arrived since the mailbox was switched originally and before it was rolled back?
The behavior is by design.
By design there is no functionality to reverse-synchronize mail from the target to the source. The undo switch mailbox option was implemented for the cases when any problems prevent the normal mail flow after switching a mailbox to the target server, assuming that eventually the mailbox will be switched again.