SES E-Mails not delivered to T-Online Emails

Hi, since about one week I encounter an issue with delivering emails to @t-online.de email addresses using SES.

I have checked the bounce notifications and the only message I got is for example the following: [messageId] => 01020184092980ec-79a3fe86-7664-4ef3-b886-fdeb373443a3-000000 [diagnosticCode] => smtp; 550 4.4.7 Message expired: unable to deliver in 840 minutes.<451 4.0.0 Unknown>

According to t-online mailserver-support this is not a t-online specific message. However there are far more messages that are not delivered which do not get any bounce feedback. So according to SES delivery is fine but the message still do not appear in the customers inboxes.

My system was sending emails successfully in the past 12 months to t-online but all of the sudden it stopped working, without any change on my side. I made many tests by sending mails from another FROM addresses and systems that use the same SES account, checked SPF and DKIM settings etc. and all seems to be fine.

I contacted the t-online mailserver-support and they can only help if I can provide an IP address or error message.

This issue seems to be online related to t-online mails as all other mail providers work fine.

BTW: when I send the exact same mail via Mailgun it seems to work. So somehow this is related to SES + t-online.

Any help would be really appreciated! Thanks a lot!