Thanks Bruce - appreciate the comment, however this wasn't the real issue. The problem wasn't being able to send from the site to multiple acocunts, the problem appears to be a conflict between forwards with the same name as full email accounts. For some reason we couldn't send to any account on the box at all. As soon as I put my gmail account into oscommerce, mail started to flow. Still can't work out what's going on for the life of me.
Cheers,
Mark