Private Relay emails bounced as 'Unauthorized Sender'

Private relay emails are not being delivered, even though we've followed the guidance here, https://developer.apple.com/help/account/capabilities/configure-private-email-relay-service/ iCloud, gmail etc. get delivered fine but as soon as its a private relay email address they get bounced as unauthorized sender.

We've tried a couple of domains but here I'll document test.x.domain.com

We have registered domains (test.x.domain.com), also the sender communication emails just to be safe (noreply at test.x.domain.com).

Passed SPF Authentication, DKIM Authentication. ESP account shows as all green checks in mailgun.

Is there any way to track down what the actual rejection reason is?

{
	"@timestamp": "2025-08-20T14:30:59.801Z",
	"account": {
		"id": "6425b45fb2fd1e28f4e0110a"
	},
	"delivery-status": {
		"attempt-no": 1,
		"bounce-type": "soft",
		"certificate-verified": true,
		"code": 550,
		"enhanced-code": "5.1.1",
		"first-delivery-attempt-seconds": 0.014,
		"message": "5.1.1 <bounce+b53c9e.27949-6qj4xaisn4k=privaterelay.appleid.com@test.x.domain.com>: unauthorized sender",
		"mx-host": "smtp3.privaterelay.appleid.com",
		"session-seconds": 1.7229999999999999,
		"tls": true
	},
	"domain": {
		"name": "test.x.domain.com"
	},
	"envelope": {
		"sender": "noreply@test.x.domain.com",
		"sending-ip": "111.22.101.215",
		"targets": "6qj4xaisn4k@privaterelay.appleid.com",
		"transport": "smtp"
	},
	"event": "failed",
	"flags": {
		"is-authenticated": true,
		"is-delayed-bounce": false,
		"is-routed": false,
		"is-system-test": false,
		"is-test-mode": false
	},
	"id": "1gtVBeZYQ0yO1SzipVP99Q",
	"log-level": "error",
	"message": {
		"headers": {
			"from": "\"Test Mail\" <noreply@test.x.domain.com>",
			"message-id": "20250820143058.7cac292cf03993f2@test.x.domain.com",
			"subject": "Test Mail",
			"to": "6qj4xaisn4k@privaterelay.appleid.com"
		},
		"size": 22854
	},
	"primary-dkim": "s1._domainkey.test.x.domain.com",
	"reason": "generic",
	"recipient": "6qj4xaisn4k@privaterelay.appleid.com",
	"recipient-domain": "privaterelay.appleid.com",
	"recipient-provider": "Apple",
	"severity": "permanent",
	"storage": {
		"env": "production",
		"key": "BAABAgFDX5nmZ7fqxxxxxxZNzEVxPmZ8_YQ",
		"region": "europe-west1",
		"url": [
			"https://storage-europe-west1.api.mailgun.net/v3/domains/test.x.domain.com/messages/BAABAgFDXxxxxxxxxxxxxxNzEVxPmZ8_YQ"
		]
	},
	"user-variables": {}
}