aneta5, that sounds odd. Have you installed all hotfixes for ePO 5.3.1? If so, you may have to do some troubleshooting from your ePO server. Also double check your SMTP setting and automatic response configuration. If you confirm everything is in order, then test to see if you are able to reach SMTP from ePO server and email successfully. You can create a whole new test automatic response or use powershell script or other utility to test SMTP. I know these questions/suggestions is not solving your problem but hopefully useful. Thanks.