we are about to switch to sending emails using SMTP. All test emails are sent ok. However, I can't really see any difference in the email headers. How can we validate that emails are really sent by SMTP. By the way, what exactly is a plugin like Postman for regarding the newsletter plugin? As far as I can see, it is not strictly necessary, is it?
Where did you set the SMTP? If you set the SMTP in Newsletter and it is active, the SMTP is used, there is no need to actually validate it. Eventually set some wrong parameter in the SMTP setting and the emails should stop.
Postman is a plugin for wordpress, there is no need to use it, it's just an alternative.