Is the OnArrival Event Sink asynchronous?

[nk997]

I need to catch all incoming emails on the exchange server and check them for bounced emails and then send them back on their way to the correct recipients. This article is useful as it provides the scripts and how to register the sink:
http://support.microsoft.com/?id=324021

However, I need to make sure that ISMTPOnArrival/OnArrival is asynchronous. I keep finding mixed information...some sites/references say that it is synchronous and some say that it can be written asynchronously. Does anyone know if the above code from the site has been written asynchronously?

Also, is there any other event sink that could be used...we've looked at onSave, but we're unsure if that will provide us with what we need.

Any help would be great...thanks.