Exchange 2000 server

Hi,

I recently set up an exchange 2000 server for a new domain. Everything is working fine but all of the sudden I get hundres of undelivered email bouncing back but I didn’t sent any of those emails. Is my mail server being relay by someone else? If so how can I track them down and what can I do to prevent this from happenning?

On my smtp server, I have set all the default setting, which allow relaying if the user can provide proper authentication, is this setting safe?