Delegate email accounts are where someone has delegated access to their email to someone else. We use it as a way for people to have access to more than one exchange account in one Outlook profile. You also need to add a POP account for sending email – note that you must set the POP account to be used for “Sending only” otherwise duplicate emails start appearing! Further details below.
The instructions below are for Outlook 2007.
Delegate access
Go to “Tools -> Account settings” in Outlook:

Outlook 2007 Account Options
Double click on the Exchange account:

Outlook 2007 Change Account Settings
Click on “More settings”:

Outlook 2007 More Settings for Exchange account
Click on “Advanced”:

Outlook 2007 Advanced Exchange Settings
Click “Add” next to “Open these additional mailboxes”:

Outlook 2007 add additional exchange mailbox
Unfortunately Outlook does not give you a search facility for finding the mailbox. Type in part of the persons name which uniquely identifies them.
Once the name has been found, click ok, and the mailbox will appear in your email navigation pane.

Outlook 2007 delegate mailbox showing in navigation pane
Sending on behalf of
If you also want to send on behalf of the other person, then set up a new pop account in the usual way, however change the send and receive settings by going to:
Tools -> Send / Receive -> Send / Receive Settings -> Define Send / Receive Groups

Outlook 2007 Send / receive Settings
Click “Edit”:

Outlook 2007 Send / Receive Account Settings
Ensure that “Send mail items” is checked and “Receive mail items” is not checked.
Click “OK”.