I have a particular end-user (User 1) who is receiving e-mails from
another end-user (User 2) within the same Exchange 2003 domain.

The problem is the following:

User 1 receives an e-mail from User 2, User 1 is CCed in this message
from User 2. User 1 wants to "Reply to All," however, User 2 does not
show up whenever "Reply to All" is clicked in any of the address fields
of the replying message. Both users are on Exchange 2003. User 1 is
using Outlook 2003, while User 2 is using both Outlook 2003 and OWA
2003.

This issue also occurs with User 2 whenever an external end-user of our
domain "Reply to All" as well. User 2 does not show up in any of the
receipiants, thus, not receiving the reply. I am not sure what server
or client the external end-user is using.

I'd like to know what's causing this issue and fix it, as in general,
most people do not look through the addresses when they click "Reply to
All" to make sure every receipiant should be there. And I already had
at least one or two instances where User 2 should have received
replies.