Jason Hendriks on "[Plugin: Postman SMTP Mailer/Email Log] WordPress (or another plugin) is currently handling email delivery, not Postman."

There are two classes of SMTP plugins. Those that use PHPMailer, bundled with WordPress:

1. WP Mail SMTP
2. WP Easy SMTP
3. Configure SMTP
4. Mail Bank

And those that re-write the pluggable wp_mail function:

1. Postman
2. Mandrill
3. SendGrid
4. Cimy Swift

None in the second class will work for you while the Supsystic plugin is installed. And it's unlikely any in the first class will solve your problem, if WP Mail SMTP couldn't.