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

There's no way to fix it. Supsystic is breaking the pluggable API.

I submitted a patch to them for their Popup plugin and they may have used it. I suggest you submit the issue to them on the Gallery by Supsystic support forum and request a similar fix.

https://wordpress.org/tags/postman-supsystic