Entourage - large list of recipients or groups are moved to bcc

[Hat]

Any of our users who are sending to a group of recipients, or a large
list of recipients:

Users enter the group/recipients in the TO field. Entourage
automatically moves them to the BCC field. This happens even if they
are inviting recipients to a meeting event.

In turn, whenever we use the BCC field - whether it's being used for a
large list of recipients or not - recipients DO NOT always receive the
message.

Any clues?

What I've done so far for some users:

Rebuilt the data base in Entourage
Rebuilt the entire account in Exchange

Nothing seems to fix this problem.

Hat