Issue
Reply mail from external to the Target is being rejected intermittently with the following error :
Your message wasn't delivered because the recipients email provider rejected it
2570410.T <<< 550 5.7.750 Service unavailable. Client blocked from sending from unregistered domains. For more information please go to http://go.microsoft.com/fwlink/?LinkId=526653. (AS2017030256) [VI1PR05MB5648.eurprd05.prod.outlook.com] [AM6PR0502CA0052.eurprd05.prod.outlook.com] [VE1EUR03FT015.eop-EUR03.prod.protection.outlook.com]
Cause
Microsoft have a rule in place that rejects email that are rewritten are not in the accepted domain list.
Solution
A case needs to be logged with Microsoft Support asking them to run the script "accepted domain recovery". Once this is run the issue will be resolved.
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