Go to your Exchange server and open up the Exchange system manager.
From here drill down to Administrative Groups>First Administrative Group>Servers>[Servername]>First Storage Group>Public Folder Store>Public Folders
This will list all of your public folders. From here you can browse to find the one that you want to assign an email address to. Right click on it and go to properties. From here click on the e-mail addresses tab. Next click on the “new” button and select “SMTP Address”. Now you can type whatever email you want in your domain and it will be delivered to this public folder for all to share.