Exchange Online / SMTP error code 550
550 5.7.1 Service unavailable, Helo domain is listed in Spamhaus. To request removal from this list see https://www.spamhaus.org/query/lookup/ (S8001) [#.prod.protection.outlook.com]
Critical severity
First, the forward DNS lookup (domain name to IP address) of your sending IP should match the HELO value set in your mailserver. Then Exchange Online checks this HELO 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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