We have an exchange 2003 sp2 cluster with win 2003 AD, users r using MS outlook 2003 client. Users are members of different groups, all the users can receive mail individually address to them & when we send a mail to a group using Outlook some users in the group wont receive the mail. Any ideas about what is causing this? Thank you.

The group you are use is Universal distribution groups ? Please reference http://support.microsoft.com/kb/271930/en-us

I've had this same issue and it always boiled down to two things: The user having a rule that moves or deletes the email OR the user themselves deleted the email (whether on purpose or accidentally).