SMTP Error message in the outbound email setup.

I'm getting the following message when testing my connection on the outbound email setup. Does anyone have any suggestions?

Error! Mailbox unavailable. The server response was: 5.7.1 Service unavailable, Client host [34.236.10.235] blocked using Spamhaus. To request removal from this list see https://www.spamhaus.org/query/ip/34.236.10.235 AS(1440) [DM6NAM10FT050.eop-nam10.prod.protection.outlook.com]