Exchange 2010. I've read about notification subscriptions, mailbox events, and EWS in Exchange and as i got it, the notiified client app have to stay connected to EWS all the time. Am i right? My task looks easy - i need to trigger some action (web call most likely) when message arrives to mailbox. Is there anything for such task in EWS 2010? Is there anything like this in latest versions of Exchange Server on-prem? For now I'm starting to think about forwarding email via trasnport rules to some unix-mta for further processing.
Yes that's correct EWS notifications require that the client application that receives the notifications stay connected if your using Stream notifications. Pull notification don't and push notifications don't but the client does need to be available to receive the heartbeat messages and then update the subscription.
There is nothing new in the later OnPrem version of Exchange there are web-hooks in Exchange Online in the Graph API