We are aware that this issue is being caused by spamrl.com, who look to be rejecting the emails.
We've noticed other providers experiencing this same issue, and have reached out to spamrl in an attempt to resolve outstanding issues.
An example of the bounceback messages being received is as follows:
550 A URL in this email (domain . com) is listed on https://spamrl.com/
. Please resolve and retry
We will continue to provide updates as the become available, our team is working on this actively to resolve any issues. Thank you for your patience and understanding.