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Just use GMX -- trying to reverse engineer why the big ugly black box that is amazon rejects some emails and not others is an excercise in frustration. Most likely their email blocking system uses some scoring methodology that mails from calibre sent via your server cross the threshold of.
I will tell you that amazon happily accepts books sent via my personal mail server (which uses postfix). But then, I have been running that server for almost two decades and it has a very high "reputation" with most RBL blacklists. And it jumps through all the dkim/dmarc/spf hoops needed for email delivery these days.
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