Click on the impersonate link next to the customer name Then on the new tab, you can see the link for users Find the user in the list find his user name Click on the impersonate link and you will be logged on impersonating the user If you need to get a password you can.
If you find your queues are getting full of NDRs from the postmaster to spammers you can turn off allow non-deliveyr reports in the Hub transport\remote domains\default\properties This however will stop NDRs from being generated that are valid. You may therefore want to configure anti spam filtering If running a single exchange server with hub.
Microsoft provide no easy way from within Exchange System manager to clear large numbers of spam emails in multiple queues. Currently you have to find the message in each queue, and delete them queue by queue. If you have-