In the newer versions of exim and also after each new upgrade of exim to a newer version, there is a problem sending emails from the servers, but also from cms platforms, such as e.g. the Opencart.
The problem occurs both when sending via smtp and via the cms php mailer. The incompatibility of exim with the ipv6 setting is the cause of the problem, so on the server where the problem occurs, we must disable ip6 support in the exim file.
The file we need to edit is located in the following path at Centos server: /etc/exim.variables.conf
Inside the file, we need to change the line from disable_ipv6=false
We restart the exim service from the server and the problem is solved.