I searched the error number at the end of the link they provided (69585) & found a google support page they locked but never addressed (figures) & I found this blog, but it doesn't really arrive at an actual conclusion as to why. It's been an issue for years apparently.
https://www.gmass.co/blog/gmail-69585-bounces/
There is, however, a comment from 2019 that said this:
"I had this happen to me and I think I found out why. It happened when I used a gmail that was over a year old, so it wasn’t due to newness I don’t think. It happened when I was trying to send it to a single other gmail address, so it wasn’t due to a large amount of bccs. By process of elimination, I figured out it was because of a particular image that I was trying to include in the email. However, it let me send other images just fine.
I believe the issue was that the particular image was from an online source which had been sent to me via email from someone else. So I converted it to PDF, then converted it back into an image, and then tried sending it, and it stopped blocking me from trying to do so."
Hopefully that helps. I have nothing else to offer if not though.
