You should be able to unsubscribe the email notification. EWS notifications are handled on a subscription basis. Typically there's one subscription per mailbox, and within the mailbox subscription you can subscribe to some or all folders.
Outside of the managed API it looks like you can terminate the subscription by returning Unsubscribe in the SubscriptionStatus element instead of OK.
In summary, there are really two ways to end a subscription on a CAS:
You can send “Unsubscribe” in the response message (as a response to a notifications message from the CAS).
You can become unresponsive to notifications messages.
We recommend the first method (sending an unsubscribe message) rather than the second method, which causes the Client Access server to perform unnecessary work.
Check this and let me know if you need further assistance. Thank you!
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