We are trying to use WebExtNotifications property on an Outlook sent e-mail, similarly to what described here: How to create a permanent notification in a new Outlook message? It works on our on-premise Exchange 2016, but not working on Exchange Online for some reason. Any reasons why? Ordinary custom properties from public strings (PS_PUBLIC_STRINGS) passing correctly.
If regular named properties survive the trip, but WebExtNotifications does not, it could be because Exchange does not want this property to be set on the incoming messages. There are quite a few properties like that - there is a whole non-sendable range for the fixed MAPI properties, and some properties are explicitly designed to be only settable on the receiving client side, e.g. the PR_BLOCK_STATUS (0x10960003) property responsible for showing the external images.
Exchange Online is newer than your on-prem server, so it is expected it will contains extra logic like this...