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:
- Check the software version of Exchange server.
- Check the Exchange Server environment settings.
- Check that the necessary applications and modules are configured correctly.
- Check for any error messages in your Exchange environment.
- Check for any misconfigured Exchange settings.
- Check the network connectivity.
- Check the DNS settings and services.
- Check the Exchange Server firewall settings.