exim rewrite and filter issues

I'm going around in circles with this issue so would be grateful for any help anyone can offer. It's probably easier to example the problem I'd like to solve first: - emails come from thirdparty1 and I'd like these to be forwarded to the original recipient and to a specific thirdparty2 address (an 'email to sms' provider) - the problem is that the thirdparty2 will require the sending email address to be from our domain Its easy enough to use the Global filters and set up each user so if the email comes in it gets forwarded to user@ourdomain.com and sms@thirdparty2.com BUT then the sender remains the original sender and the thirdparty2 doesn't accept it. I've Enabled Sender Rewriting Scheme but that doesn't help on its own I've even added an exim rewrite similar to "info@thirdparty1.com "Email" fw" which rewrites the sending address but the filters don't seem to work at all in that case. I've spent hours going around in circles and testing this but I must be missing something - any help would be great, thanks in advance.