[Exim] Filters - mail command

I have a set of filters that extract the subject and sender and send them to a mobile network for a poor-mans SMS notification. Unfortunately, they recently altered their network so that blank senders can only very recipients, not actually send data. The 'mail' filter command lets me do nigh anything I want with the headers, but appears to send the message using a blank envelope sender. Is there a snowballs chance I'm missing something or that this would be easy to fix?