You have to add a new field to ir.mail_server  model in order to know which server is for mass mailing: 
mass_mailing_server = fields.Boolean()

Then override method connect(....)  and change the code where is searching for the first server available.

mail_server = smtp_encryption = None
if mail_server_id:            
​mail_server = self.sudo().browse(mail_server_id)        
elif not host:            
​mail_server = self.sudo().search([('mass_mailing_server', '=', False)], order='sequence', limit=1)

This is tested on v14.

Did you already find a solution for this? I am having the same problem