1.) Can configure different mail server for mass mailing
Mass mailing server can be different from default Odoo mail server (for example smtp.mandrillapp.com). If document model is mail.mass_mailing.contact email is sent through mass mailing server.
2.) Can configure different mail server per user
User can use own mail server. Mass mailing server is still used for mass mailing.
3.) Mail server priority
If several mail servers are defined use mail server with higher priority (lower number = higher priority)