In this case previous error should be “Connection timed out”.
H=reception.mail-tester.com [94.23.206.89]: SMTP timeout after initial connection (4660 bytes written): Connection timed out
Similar error where here: Setting SMTP2GO as relay - #6 by BrandCom
And the solution based on that topic is opening port 25. This shows the same for me, that e-mail forwarding is using port 25 instead of global or domain based SMTP relay. Am I right?