Subscribing to new mail event of multiple impersonated user through one ExchangeService object

Question

I am able to subscribe to push notifications by specifying impersonating users' credentials and then specifying impersonated user's email in ExchangeService' ImpersonatedUserId property.
However want to know how can I handle new email events on multiple email ids that are impersonated by one email id, since there is no collection as such on ExchangeService to specify multiple Impersonation Ids. In fact I think we can handle only one email
id by using single ExchangeService object. And to subscribe to multiple user mail events, even through impersonation, I have to create multiple instances of ExchangeService. And also multiple callback listeners too. This is what I want confirmation about:
Q. Do I need to create multiple instances of ExchangeService and callback listeners to handle new mail event on multiple user id with impersonation?
Q. Without impersonation?
Q. Also want to know given credentials of impersonating user, can I retrieve list of all users impersonated by that user.

Answer 1

> Without impersonation?
You don't need a separate instance in this case (you need separate subscriptions for each mailbox). However there are throttling limitations around the total number of subscriptions one particular user can make see http://msdn.microsoft.com/en-us/library/office/dn458788(v=exchg.150).aspx
For
subscribing to mailbox of same user id, we can use same object of ExchangeService. However I did asked for subscribing to mailboxes of
different user ids (or simply multiple mailboxes) without impersonation.
I cannot find how can I specify credentials for multiple mailboxes on single instance of ExchangeServer. Just following:
service.Credentials = new WebCredentials(userid, password);
 Is it really possible to use same instance ExchangeService to subscribe to multiple mailboxes?