[Message blocked but IP is not on blacklist]

Thank you for your quick reply!

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who is stating it is?

The email admin at the company is saying that no one at their company is receiving our email newsletter — I confirmed it in our error logs (see attached). 

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Sometimes a "clever Trevor" have a blocklist working in reverse?
Or it could be a fake bounce from someone you are mailing too?

I'm not sure what you mean by this. 

[Message blocked but IP is not on blacklist]

I'm seeing this error for all recipients on a particular domain:

"spamcop.mimecast.org Blocked - see https://www.spamcop.net/bl.shtml?198.61.254.91. - https://community.mimecast.com/docs/DOC-1369#550 [bEjCcA39P3SxsOV3CZ9qSw.us331]"

However, as you can see here, that IP is not on the blacklist: https://www.spamcop.net/w3m?action=blcheck&ip=198.61.254.91.

Here's a link to the message and headers: https://www.mail-tester.com/test-iyn5n@mail-tester.com