It may be worth pointing out that this will also prevent the mailbox from receiving email
I dont think Japes is correct in that hiding the address from global list will prevent that mail box from receiving email. Maybe Technochic can explain this in more detail as she appears to be really knowledgeable about this stuff. I just know that we have done this with customers with no affect on their ability to send or receive email. To my understanding, it is an attribute and not a functional setting.
Dwiebesick is correct on this one. What we have found in practice however is that when we hide a mailbox from the GAL, everyone who previously was able to send to it still can, but new users who may learn of it and try to send to it cannot. So if it is a new mailbox we let it populate to the GAL first, then have everyone who NEEDS to send to it send an email, then we hide it again. I do not know if this is still true since we moved to Exchange 2007. I have not had to try it yet.