I'm seeing multiple bounces on emails sent to NTLWorld accounts. They all show the same "Message expired: unable to deliver in 840 minutes." error.
Messages sent use DMARC, DKIM and SPF. The messages are not spam and sent to members who have requested them.
For example, I got this bounce message from Amazon Web Services:
{"notificationType":"Bounce","bounce":{"feedbackId":"010b018f67ca9d4c-f756af4c-1e5f-4cef-bd1f-2ec1ed06b054-000000","bounceType":"Transient","bounceSubType":"General","bouncedRecipients":[{"emailAddress":"Graham Anderson ","action":"failed","status":"4.4.7","diagnosticCode":"smtp; 550 4.4.7 Message expired: unable to deliver in 840 minutes."}],"timestamp":"2024-05-11T13:14:42.000Z","remoteMtaIp":"84.116.6.18","reportingMTA":"dns; d218-1.smtp-out.eu-west-2.amazonses.com"},"mail":{"timestamp":"2024-05-10T22:36:04.640Z","source":"Andy Henderson ","sourceArn":"arn:aws:ses:eu-west-2:658229544723:identity/portsmouthctc.org.uk","sourceIp":"82.165.84.93","callerIdentity":"root","sendingAccountId":"658229544723","messageId":"010b018f64a632a0-2da72ef1-b2ea-44ab-8da6-319f5a9bcfa2-000000","destination":["Graham Anderson "],"headersTruncated":false,"headers":[{"name":"Date","value":"Fri, 10 May 2024 22:36:04 +0000"},{"name":"From","value":"Andy Henderson "},{"name":"Message-ID","value":""},{"name":"X-Mailer","value":"PHPMailer 6.9.1 (https://github.com/PHPMailer/PHPMailer)"},{"name":"MIME-Version","value":"1.0"},{"name":"Content-Type","value":"multipart/alternative; boundary=\"b1=_wxNfqdlCrshykVN0MEsgO5lNKuSKIdL2bCmgJEPQAQ\""},{"name":"To","value":"Graham Anderson "},{"name":"Subject","value":"PCTC Update - 10 May 2024"}],"commonHeaders":{"from":["Andy Henderson "],"date":"Fri, 10 May 2024 22:36:04 +0000","to":["Graham Anderson "],"messageId":"","subject":"PCTC Update - 10 May 2024"}}}