Apple / SMTP error code 550
smtp;550 5.7.1 Your email was rejected due to having a domain present in the Spamhaus DBL -- see https://www.spamhaus.org/dbl/
Critical severity
Apple Mail checks all URLs in your message content against the public block list Spamhaus. Spamhaus is a very reputable block list with few false-positive listings. You must cooperate with Spamhaus volunteers to resolve the listing. Remember that even once the listing is removed, it may take receivers up to 24 hours to refresh their cache and to see delivery resume.
Documentation
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