Email Routing: 550 5.0.350 Remote server returned an error

I’m receiving 550 errors when sending from Office 365 through to a domain hosting on Email Routing.

Error Details
Reported error: 550 5.0.350 Remote server returned an error → 521 554 Upstream error. hYo0pHbpThyD


DSN generated by: PS1PR03MB4843.apcprd03.prod.outlook.com

Message Hops
HOP TIME (UTC) FROM TO WITH RELAY TIME
1 8/02/2022
12:21:17 am TYZPR03MB6021.apcprd03.prod.outlook.com TYZPR03MB6021.apcprd03.prod.outlook.com mapi *
2 8/02/2022
12:21:17 am TYZPR03MB6021.apcprd03.prod.outlook.com PS1PR03MB4843.apcprd03.prod.outlook.com Microsoft SMTP Server (version=TLS1_2, cipher=TLS_ECDHE_RSA_WITH_AES_256_GCM_SHA384) *