[Xymon] Changing sender of alert emails?

Hi Folks -

I have 2 instances of Xymon running (one internal to my network and one external). We have decided we want to set up the ability to ack over email. This is very easily set up (already did it on the internal version), except for one small detail, email from BOTH comes from xymon at mydomain.com. This means that replying to this address can only go to one or both. Both kind of stinks, as it means one system will be getting bad ack's all the time (and then xymond will go yellow). I'd like to set the email sender on the external instance to something different, but can't seem to find it anywhere in any of the configs. I looked through alerts.cfg and the man page and nothing stood out to me.

Is this possible?

Thanks! Skadz