Outlook / SMTP error code 550
smtp;550 5.7.1 Service unavailable, MailFrom domain is listed in Spamhaus. To request removal from this list see https://www.spamhaus.org/query/lookup/ (S8002) [#.eop-nam02.prod.protection.outlook.com]
Critical severity
The MailFrom domain is also known as the Return-Path header domain, and is used to validate SPF authentication (it's not always the same domain as your From address for the message). Outlook checks your MailFrom domain 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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