Reporting-MTA: dns; bittern.ukcod.org.uk Action: failed Final-Recipient: rfc822;xxxx@xxxxx.xxx Status: 5.0.0 Remote-MTA: dns; mssc-org0e.mail.protection.outlook.com Diagnostic-Code: smtp; 550 5.7.51 TenantInboundAttribution; There is a partner connector configured that matched the message's recipient domain. The connector had either the RestrictDomainsToIPAddresses or RestrictDomainsToCertificate set [DB5EUR01FT003.eop-EUR01.prod.protection.outlook.com]