Exchange Online / SMTP error code 550
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)
Critical severity
Exchange Online checks the sending IP 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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