I'm having the same sort of trouble, but a little different. We have group email addresses and a contact form we have on our site (www.adam.co.uk) - it flings emails out to everyone in the group, even though only one email is used. We're having some mailer daemons ping through to those people in the group who have a slightly different email designation/ending.
For example this is everyone in this info@adam.co.uk group:
Everyone received the email who has the 'adam.co.uk' designation, but anyone who has another (in this example 'centerprise.co.uk') gets a daemon. Does anyone have any ideas why this might be?