I use the following construction:
One LINUX proxy fetches mails for all users from one POP3 account and
forwards them to the Groupwise 6.5 server via smtp.
Groupwise distributes the mails to the recipients.

If one mail comes to several users via blindcopy bcc, no one of the users
will receive this mail. It will be sended to postmaster as undeliverable.

What goes wrong?