Exchange Online an email provider, headquartered in Washington as part of Microsoft. It's offered by both Office 365 and Microsoft 365, providing businesses with spam filtering, block lists, and firewall protection. If a message is going to spam or getting blocked, there are typically instructions for the email admin to fix the specific error in Exchange documentation.
smtp;550 5.7.1 Service unavailable, Client host [x.xx.xx.xx] blocked using Spamhaus. To request removal from this list see https://www.spamhaus.org/query/ip/0.0.0.0 (AS16012611)
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]