send to groups from outlook/exch5.5

We use MS exchange 5.5 internally via outlook 2000. We use MAILTRAQ to collect & deliver POP3 mail for users externally, users setup for exchange internally & internet mail for external addresses. Indiv mail is distributed correctly, however, when we send to a defined group and all recipients are external - it sends ok to external recipients. When any of the group recipients are internal recipients, all external recipients are ignored and all is sent to Exchange and external addresses discarded! what should I look for in Exchange? - it worked before!