Question about 550 Error spam link with outgoing Email (NameCrane/SpamExperts)

Hi,
I am experiencing an issue that I have never had happen to my emails before.
I am with NameCrane EU servers with SpamExperts.

I am replying to an email address that I regularly send and receive emails to, and I got a non-delivery message with error 550.

I couldn't figure out what it was at first, but I found that the image in the original sender's email was linked to another domain for hosting/tracking.

When I removed the images from my new reply message, the message went through fine.

Just so I understand correctly, is this a SpamExperts feature?

I contacted the recipient, but they said that they have been sending and receiving emails all day, they just can't seem to be able to send me emails today, and I cannot reply to their emails.

I did a blacklist search, and the domain which is hosting their email image signature is indeed on a block list, but why is only SpamExperts blocking it?

I will copy and paste the full error and domain when I get into my other account.

Would it be wise for me to whitelist their entire domain for incoming and outgoing emails?
Thanks