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There are various things which may be causing this. I noticed that the message headers you posted including “X-Backscatter: Yes”, which means that the Fastmail backscatter spam filter settings will affect your results. We also don’t know the target you are using for that alias, and that will affect the delivery.
I will repeat my suggestion from earlier in this thread. If you host your domain at Fastmail and have the wildcard alias enabled, all you have to do to block one alias is to create an alias at your domain in the Fastmail Aliases screen and then disable that alias. This will result in Fastmail not accepting incoming mail for that alias.
Bill
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