Outlook.com?
Could that account for my inability to send email to an NHS address on April 29th? Or indeed the message that took about 24 hours to reach me from a friend on May 2nd/3rd, with most of that time spent on outlook.com servers?
The bounce from the NHS mail contained full diagnostic information. It was in a mail loop at outlook.com:
Received: from AM6PR10CA0087.EURPRD10.PROD.OUTLOOK.COM (2603:10a6:209:8c::28) by AM5SPR01MB03.EURPRD10.PROD.OUTLOOK.COM (2603:10a6:206:1b::28) with Microsoft SMTP Server (version=TLS1_2, cipher=TLS_ECDHE_RSA_WITH_AES_256_GCM_SHA384) id 15.20.1835.13; Mon, 29 Apr 2019 14:44:48 +0000
Received: from HE1EUR02FT049.eop-EUR02.prod.protection.outlook.com (2a01:111:f400:7e05::207) by AM6PR10CA0087.outlook.office365.com (2603:10a6:209:8c::28) with Microsoft SMTP Server (version=TLS1_2, cipher=TLS_ECDHE_RSA_WITH_AES_256_CBC_SHA384) id 15.20.1835.12 via Frontend Transport; Mon, 29 Apr 2019 14:44:48 +0000
Received: from EUR04-HE1-obe.outbound.protection.outlook.com (104.47.13.51) by HE1EUR02FT049.mail.protection.outlook.com (10.152.11.8) with Microsoft SMTP Server (version=TLS1_2, cipher=TLS_ECDHE_RSA_WITH_AES_256_CBC_SHA384) id 15.20.1835.13 via Frontend Transport; Mon, 29 Apr 2019 14:44:48 +0000
Received: from DB6PR1001CA0034.EURPRD10.PROD.OUTLOOK.COM (2603:10a6:4:55::20) by VE1PR10MB2879.EURPRD10.PROD.OUTLOOK.COM (2603:10a6:803:10f::28) with Microsoft SMTP Server (version=TLS1_2, cipher=TLS_ECDHE_RSA_WITH_AES_256_GCM_SHA384) id 15.20.1835.15; Mon, 29 Apr 2019 14:44:46 +0000
Received: from AM5EUR02FT030.eop-EUR02.prod.protection.outlook.com (2a01:111:f400:7e1e::209) by DB6PR1001CA0034.outlook.office365.com (2603:10a6:4:55::20) with Microsoft SMTP Server (version=TLS1_2, cipher=TLS_ECDHE_RSA_WITH_AES_256_CBC_SHA384) id 15.20.1835.12 via Frontend Transport; Mon, 29 Apr 2019 14:44:46 +0000
Received: from EUR03-VE1-obe.outbound.protection.outlook.com (104.47.9.54) by AM5EUR02FT030.mail.protection.outlook.com (10.152.8.180) with Microsoft SMTP Server (version=TLS1_2, cipher=TLS_ECDHE_RSA_WITH_AES_256_CBC_SHA384) id 15.20.1835.13 via Frontend Transport; Mon, 29 Apr 2019 14:44:46 +0000
Received: from AM6PR10CA0049.EURPRD10.PROD.OUTLOOK.COM (2603:10a6:209:80::26) by HE1PR10MB1548.EURPRD10.PROD.OUTLOOK.COM (2603:10a6:7:5d::25) with Microsoft SMTP Server (version=TLS1_2, cipher=TLS_ECDHE_RSA_WITH_AES_256_GCM_SHA384) id 15.20.1835.13; Mon, 29 Apr 2019 14:44:44 +0000
Received: from VE1EUR02FT014.eop-EUR02.prod.protection.outlook.com (2a01:111:f400:7e06::208) by AM6PR10CA0049.outlook.office365.com (2603:10a6:209:80::26) with Microsoft SMTP Server (version=TLS1_2, cipher=TLS_ECDHE_RSA_WITH_AES_256_CBC_SHA384) id 15.20.1835.12 via Frontend Transport; Mon, 29 Apr 2019 14:44:44 +0000