We have exactly the same problem as described in
in our Groupwise 2012 System (12.0.2-112085)
If we send a mail to a large distribution list with an attachment it
works up to specific users. These users forward the mail to an external
address and purge the message then immediately (rule based).
The remaining members of the list don't get the message any more, the
post office throws the 8209 error for all the following recipients.

This happens not always, but intermittently.

Do I have the chance to solve the problem from the server-side or is it
necessary to remove the delete-rule in the users-mailbox?

thanks in advance