Is there a way to alternate between different outbound mail servers?
My proposal is using CiviCRM Extension like this one Alternate Mailing Server.

Assumptions:


each alternate server has own setting mailing_backend_alternate[1-9],


first at mailing_backend_alternate1,
second at mailing_backend_alternate2,

in session variable is stored which server will be used, 
server is changed by hook_civicrm_alterMailer().