If you are trying to send an e-mail from a shared mailbox or as a different user in Outlook 2010, you will first need to have delegate access to the account you intended to send from. Once you have this, you can send e-mails from their account by opening a new e-mail message, clicking the options tab and then click the "From" option.
Once this option has been selected, you should have the ability to choose which e-mail address to send from. Click the "From" box above the "to" and "cc" boxes and click either the address you would like to use or the other e-mail address option.
Type the address you would like to send from into the box and click OK. If it works properly then you will be able to send from this address in future. If there are still issues then check the e-mail address is spelt correctly, otherwise you may need to call us to check the permissions that you have available.