This leads me to suspect that your issue has more to do with the PHP mail() function used by the plug-in than the functioning of the garbage email address.
I have in the past run into a similar problem, where the PHP mail() function was supplying my DreamHost username and server as the 'sender' address, and replying to such emails resulted in them being sent to my default DreamHost email account. The thread below has a little more information on this.
If you have the ability to modify the plug-in, you might be able to track down where the mail() function is being used and add the required '-f' flag, to force the sender (as detailed in the linked thread).
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