Exchange and Outlook Problem

We have the serious problem about exchange 5.5. Please help
Environment
Server Platform: NT4.0 with Sp6a, Exchange 5.5 with SP 4.
Client Platform: win2000 pro with sp4, Microsoft outlook 2000 with sp

Case 1)
Two of internal user sent mail to other internal user have the fill in TO,
CC and BCC.
However, only the user at BCC field can receive this mail. TO and CC user
never receive.
I check the exchange server's event viewer. i only can see the transition to
the user at BCC field and not any information is provide about the
transaction at the field TO and CC user is submit or not.
Sender also not have any alert message from server. Finally, at TO or CC
user find out mail is not receive then report to sender. Sender report to us
then we try to ask sender submit again(Method: open new mail then copy past
the send item content.) All receipts is receive immediately.

Additionally, when we open the mail at sent item about this problem mail. we
find the BCC will disappear when we click the BCC user. next time re-open
this mail BCC user will re-appear but it must disappear when we click the
BCC Field.

John Smith