When you add a new user and their email address doesn't show up in the Global Address List, work through the tips and suggestions in the ever-useful Exchangepedia Blog: http://exchangepedia.com/blog/2005/11/new-user-does-not-show-up-in-gal.html
Bulk-add new users with the script here (http://www.exchangepedia.com/blog/2006/11/exchange-server-2007-bulk-creation-of.html
) or give existing users mail access (http://exchangepedia.com/blog/2006/12/bulk-mailbox-enabling-users-exchange-shell.html
- this also has details for adding multiple meeting rooms as resources). And when an employee gets replaced, the steps at http://exchangepedia.com/blog/2006/03/how-to-reconnect-mailbox-to-another.html
make it easy to connect the previous user's mailbox to the new user account so that mail for that job role keeps going to the right place.
Reject spam with a custom message
Just in case the message you're rejecting comes from a real person, you can have Exchange send a less cryptic message that the default error; it will make your customers look more professional. This blog post http://mostlyexchange.blogspot.com/2006/12/exchange-2003-sp2-imf-tuning.html
explains how - but make sure to keep the 550 error code at the beginning.