How to use different mail interfaces (Mail System)? I have a newsletter (simplenews) being used from time to time and it uses SMTP server to send mail. 
All other emails from my site should be sent using default interface. How do I do this?

I thought Mail System module could help me with this, but seems I'm doing that wrong thus I can only switch between interfaces, but not assign SMTP to simplenews only and left other using default one. How can I do that?
For D7 you can define a custom mail system using the $conf['mail_system']

$conf['mail_system'] = array('mycustom_module_thekey' =&gt; 'MyCustomEmailSystem');


Then you can use the drupal_mail function

drupal_mail('mycustom_module', 'thekey', $email, language_default(), $params, $from);