In an organization, staff come and go. For smooth transition when a staff leaves for greener pastures in Murang'a, it is easier to use designation-based e-mail addresses. However, for personalized handling (say of external customers)  first.second@xyz.com seems comforting, but in a service industry, when first.second leaves, they're likely to leave with your clients, if they liked him/her! Actually, I see not much advantage in either of the two :-)