Email sent to ISP always bounce with timeout

I successfully send about 12,000 emails per day to different subscribers via SES. For about 2 weeks now, I have been receiving soft bounces from an ISP (World4You) for all delivery attempts with a timeout after 840 minutes. Is there a way to examine the connection to the ISP’s SMTP server more closely? The SNS topics for bounces are not informative enough. The ISP’s feedback is always that they can’t see any delivery attempts for claimed email addresses in their logs and that I should look for errors in my logs during the connection setup. There is a similar post where the problem occurred with the ISP ‘t-mobile.de’ and throttling by the ISP was responsible.

Important parts of the Bounce notification: Bounce Type: Transient Bounce Sub Type: General Remote MTA IP: 81.19.149.68 Diagnostic code from bounce: smtp; 550 4.4.7 Message expired: unable to deliver in 840 minutes.<421 4.4.2 Connection timed out> Reporting MTA: dns; b224-17.smtp-out.eu-central-1.amazonses.com

Thanks and regards, Christoph