Hey, thank you for your reply.
My first guess ist that the chain is not correct and so the server's certificate cannot be validated.
you are correct about it. I read the openssl output wrong, i got confused by "Verify return code: 0 (ok)" and thought the connection was successful. But is was not.
If you are sure that your PHP script connects to the right host then you can remove temporarily validation just to see if this will work. If so you should enable validation again and examine the chain on your mailserver.
Yeah, i think this is (to my surprise) not a PHP issue.
