I recently reset my WP8.1 Lumia 1020. Previously I had several Google email accounts and one Exchange email account (for work, who run their own private Exchange server). All of these were set to sync email "as items arrive".
As I am adding all my email accounts back, the Exchange one no longer lists "as items arrive" as an option in the list. It seems crazy the one account that has a problem is part of MS' own ecosystem!
What's gone wrong and how do I fix it?