Welcome to the forums and thank you for your question. I have a few questions to understand the situation better...

Firstly what version and build of GroupWise is installed on your servers?
Is this distribution list [group] a GroupWise group or is it an LDAP associated group?
I take it that members of this group reside on the Post Office not receiving the email?
Set the log level of all your POA's to Verbose, send an email to the group and look for any errors.