If there are any issues on mail forwarder, first thing to check if the DNS is setup correctly. You can use a tool like the command line tool dig or just use https://intodns.com.
If you see the DNS is setup correctly, now check if the MX records has been setup correctly. You can use the free and public tool for this: https://mxtoolbox.com
In this example I use powerpanel.io
It will give you a overview of the MX records found. Also it resolved to the IP addresses. Please check the IP address if it is listed the correct server. Here you find the overview of our public ip addresses.
If DNS and MX are not setup correctly it will give you the below output:
If DNS has setup probably, the we need to check if the SMTP server is responding. In the online tool you can easly click the SMTP test link.
Success output looks like this. So this also good.