Mail marked as spam


I am trying to forward an email that I got, but keep getting “the email could not be sent”, and NDR email from Mailer-Deamon.
“I am deeply sorry, but I was not able to deliver your mail to the following addresses:”

If I send a normal email, it goies through.
If I save the email I want to forward, as a PDF and attach it, it gets marked as spam…

Tried sending to both the person I intended, and to my address, both got blocked.

In the end, I had to send the PDF saved email from address.

I tried looking through my logs for an explenation, but I cant see why it thought it was spam.