Domains that have not configured anti-phishing/malware policies in O365 to allow ExchangeDefender networks are seeing inbound delays when not properly configured for ExchangeDefender
451 4.7.500 Server busy. Please try again later from [65.99.255.114]. (S77714) [CY4PEPF0000EDD5.namprd03.prod.outlook.com 2026-06-03T13:47:44.869Z 08DEBCEBD2EAE39F]
In order to resolve the issue, ensure the domain is fully deployed according to our O365 Deployment Guide, specifically M365 Tenant Anti-phishing and Anti-spam Settings