This configuration is specific and used especially when you need to send a mass mailing with multiple SMTP relais.
Most of mass mailing application accept only one SMTP server to send the mass mailing.
So you cannot define multiple SMTP server with different settings and behavior.
For other things it sould be intersted to load-balance SMTP flow with multiple SMTP relais.
With Artica 1.4.122100 you can provides this architecture in an easy way.
