have you tried mail-tester.com?
Send an email to the one it provides in as close to the format as one that was marked spam, so the content is the same and you should get a result.
Hi
All of a sudden I am getting reports from clients about receiving duplicated emails in the hundreds with the same Message ID.
We have recently introduced greylisting to solve a problem where the server was blocking emails with secureserver.net in the SPF record and the emails that were...