Exchange 2003 multiple domain (AD & SMTP) distribution groups

We have one AD forest with 3 domains and one Exchange org.
The RUS stamps accounts based on information store with the appropriate smtp addresses.
Users can send internally and externally correctly.
The problem is in sending to a distribution group across domains.
I can email from the top level domain to any of the other domains, but I cannot mail from any other domain to either top level or peer level domain.
I get the following error:
The following recipient(s) cannot be reached:
     <distribution list name> on 2/12/2008 12:33 PM
            You do not have permission to send to this recipient.

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VahikCommented:  read this carefully ....specially the part about expansion server and global distribution groups...,289142,sid43_gci1119791,00.html
CG_HDAuthor Commented:
Mail enabling the security groups fixed the problem.

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