If you have 15 different email addresses 1 with each company and you want all mails to go into 1 mailbox do the following...
- Open ADUC
- Find your account
- Double click your account, to view its properties
- Click on the Email Address Tab
- Click New, Select SMTP
- Add the email address to this account
- Continue the process until you have added all of the addresses to this account
Then you can open your mail client and set up rules to forward email from a particular domain into a specific folder
If you are looking to setup multiple domains in an Exchange Server environment you will need to modify the Recipient Policy.
http://support.microsoft.com/kb/822447Hope this helps~!