Unfortunatly I did not find any solution yet. I opened a support ticket at adobe.

Thx for the reply. How do I know if a server certificate is trusted by the AEM instance? Can I test that somehow? So the guys who are running the SMTP server have to change their certificate? If yes, to which one? It seems that they are using a Let's Encrypt certificate: Common name: R3 Organization...

I have an issue when sending out mails in AEM trough an external SMTP with SSL on AEMaaCS. On my local development environment, the mails are sent without any issues. The error on AEMaaCSCaused by: org.apache.commons.mail.EmailException: Sending the email to the following server failed : proxy.tunne...