I was hoping that would be something you could answer :)
I checked the details of the SMTP plugin and it says it passes all emails that use the wp_mail() function: their FAQ says
If other plugins you're using are not coded to use the wp_mail() function but instead call PHP's mail() function directly, they will bypass the settings of this plugin. Normally, you can edit the other plugins and simply replace the mail( calls with wp_mail( (just adding wp_ in front) and this will work.I checked your plugin and that appears to use wp_mail() so shouldn't fall foul of this restriction - however, it seems like this is the most likely cause of the problem?