Here is a list of steps you can take to try to resolve issues with emails not sending when replied to or forwarded in Exchange:

1. Check the software version of Exchange server.
2. Check the Exchange Server environment settings.
3. Check that the necessary applications and modules are configured correctly.
4. Check for any error messages in your Exchange environment.
5. Check for any misconfigured Exchange settings.
6. Check the network connectivity.
7. Check the DNS settings and services.
8. Check the Exchange Server firewall settings.