Hi,
There is no limit. When a user forwards the mail and the next receiver opens the mail as well Acymailing records an additional opend statistic at the original user. So your open rate doesn't increase, since that statistic is looking at unique opens.
Reason for this is that the statistics are matched with your subscriber and that subcriber id doesn;t change when the mail is forwarded. Furthermore the person who receives the forwaded mail is not in your database, meaning that opens can not be registered other then as an extra open at the original receiver.
