Are these two independent installations of EventSentry (full install) or are these machines with just the agent installed but on different networks than the central console? Are you using the collector? If you click on your email action, if the "Use Collector" is checked then you're using it. If you are using the collector, could you look in the application log on the central management console machine for any of these errors:
You can also look on the remote host in the application log for any 905 events. If you find any of these, could you please let us know what they are?