Hi, I really hope someone can help me, i've looked everywhere for a solution and its almost as though i'm the only person to ever have seen this fault!

We have exchange 2003 (Fully patched-Native mode), all works well except emails sent to distribution lists from an external email address.

These emails work so long as the list(group) has 14 users or less. If I email the group from an internal address there are no issues.

If I add a 15th user to the group the mail fails, it will send to 2-3 members of the group and no one else. (I see this through message tracking and have confirmed it by checking mail boxes.)

We have a mail gateway in front of the exchange server which filters spam using Open Relay Filter (ORF) and Sophos puremesaage/antivirus on the exchange server itself. (Exchange is also the PDC) I assume the gateway isn't an issue as Exchange is the server expanding the group and if i email the group recipients as individuals the mail goes through.

I have made sure the group is universal and checked all the size/connection limits on the SMTP connectors.

I'm out of ideas. Can anyone help?

Thanks for reading.