You cannot type directly in to a lookup field.
You user would need a Table Permission that provides enough privileges to read, create, and append/append to (I can never remember which one it should be for append/append to).
On the basic form add a metadata record for the lookup attribute, set the Basic Form for Create field. to a basic form that will let them create the email address record When they open the Lookup control on the portal they should then get the list and also a 'New' button.
