It is possible that this is related to your sending service, because, for example, I don't have this when I am using my SMTP server, but ....

The preview text is something we add at the beginning of the email, and then we hide it, so it is not displayed when looking at the email, but it is displayed when you're looking at your inbox

So that's normal that is shows "Invisible text"
There are 2 solutions then:

  • People ignore this (I don't think this will make email be considered as spam for this simple thing)
  • They can remove the preview link

I hope that this explains it. As you can see, it is actually quite safe to ignore this spam test report line