Account screenshot

Have you asked optimum exactly why they are rejecting your email? Perhaps when they tell you you could share their answer.

I have sen rejections from SMTP server because the folk running it are disallowing sending email from anything but their webmail because competence is not high in the technical departments of many ISP's.

I have also seen email rejected because it has something like a facebook logo downloaded from the internet in a signature that has been identified in SPAM emails previously.

SPAM detection is like a free for all. Lots of ideas. Mostly bad and everyone is doing their own thing. Generally without thinking to much about repercussions. SO as only your provider can tell you exactly what this ID 661FD756017B2BC6 means to them, I suggest you ask them to explain.