Exchange 2010, External Email and Distribution Group issue
We have an Exchange 2010 server and I am trying to get a specific email address to forward any incoming email to a distribution group. I have set up the forwarding happily through the management console so it redirects to the created distribution group. The distribution group consists of internal and external contacts. I have created the external people as mail contacts and I can send them an email by using this mail contact in the normal way but when they are part of a distribution group it fails to send, however the internal contacts work. We have other external distribution groups and we can send emails to these external addresses happily but it seems the forwarding element causes an issue.
I have looked at the Message Delivery Restrictions on the email address, mail contacts and distribution group and ensured the require that all senders are authenticated is unticked but it still won't send to external mail contacts.
Have I missed anything?