Why is outlook sending me failure to deliver when I send emails to my own address

[Microsoft Windows]

I send myself a message and I get this post failure message. Delivery has failed to these recipients or groups:robroy_walton@hotmailYour message wasn't delivered because the recipient's mailbox is undergoing maintenance and can't accept messages now. Please try resending the message later.Diagnostic information for administrators:Generating server: BN6PR19MB0116.namprd19.prod.outlook.comrobroy_walton@hotmailRemote server returned '501 5.1.6 Recipient addresses in single label domains not accepted [SJ0PR19MB4446.namprd19.prod.outlook.com 2024-12-26T01:09:58.655Z 08DD249830196F54]'Original messa

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