Mail delivery failed: returning message to sender

We can't help you with this. The header already clearly states what is wrong and what you have to do.

Your server ip is on an RBL blacklist. Follow the csi.cloudmark.com link to try and get your ip delisted.
Hello Richard,
I have done the reset process. By any chance do you know the time it will take to reset?
 
No unfortunately not. I have been contacting several lists in the past 15 years, but didn't have to do with cloudmark.
However, if your system isn't spamming I presume it will probably take no longer than 48 hours (workdays).
 
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