inability 2 send distribution list

I can create a distribution list with recipients from multiple domains or enter their addresses thru Outlook (separated by semi-colons) and the first two will go and the rest will sit in my Exchange 5.5 queue indicating Host Unreachable. Then, if the "Next Retry" times are identical, they will expire without sending. But, if I select them individually and hit "Retry Now", they will send. This all runs thru a PIX firewall, but it has 100 addresses available thru the local pool. Please advise...........thanks