I'm sorry to hear you're experiencing problems with sending mail to that recipient. The error suggests that the recipient or its provider has blocked you either via domain or the Amazon sending end-points. I would recommend reaching out to the recipient to see what is blocked.
Kind regards, Robin
ps. If you're using the complementary alias.awsapps.com domain I would recommend to switch to a personal registered domain as awsapps.com is only intended for testing purposes.
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