It seems that by default, it is set to reject mails with a spam score of more than 20 at SMTP time. How to change the value of spam score for rejecting the mails at SMTP time ?
You can do that by doing the following :
1. Go to WHM >> Main >> Service Configuration >> Exim Configuration Editor
2. Look for the section ‘ACL Options’ .
3. The first parameter under that deals with the spam score setting at SMTP time. De-select the check box corresponding to that.
4. When you de-select the check box, it will give you the other available options. Check the box corresponding to the desired value.
5. Click on ‘Save’ at the bottom to save the settings.