if you are using Outlook 2007 or 2010, you can have multiple email accounts configured within your client. If each account is a different mail box, i.e is myname@anotheraccount.com and myname@myaccount, you should have the ability to select these address from the "from" field when composing a message.

Note: This works best if your primary account is an exchange account. however, this does not disallow the above steps to be used.

-Check to see if all of your accounts are configured.
- Compose a message. in the from field it will default to your defaulted account. You should be able to select the From button to select additional accounts to send from.

hope this helps