which transport event sink shall i impletement if I want to catch the email just before it is delivered to local drop folder?

I implemented the IMailTransportSubmission event interface but I think it is not the final step before the email is dropped into the local folder. There are so many transport events: routerengine, routerreset, postcategorize...etc, I cannot find a clear picture about all the events and don't know which one is the 'final step' before the local delivery. Please give me some light, thanks a lot for your help! :)