In Apple's Mail application you can organize incoming mail messages by setting up rules to filter and place messages in mailboxes, run scripts, or forward messages to different accounts.
A few people who have upgraded to Lion have experienced a problem in the way message forwarding is handled in Lion's version of Mail, where the message text is sent but attachments that should be forwarded along with the message are being left behind.
MacFixIt reader "Pierre G." wrote in describing the issue:
Some people have also found that despite this problem happening with rule-based forwarding as Pierre G. describes, manually forwarded messages include attachments as expected, indicating the problem is based solely in the use of rules and not in the management of attachments. As a result, one possible explanation for this could be that the message attachments have not fully downloaded to your computer by the time the rule runs and sends the message off again.
To test this, disable the rule in Mail's preferences (uncheck it), and when a message arrives make sure the attachment is fully downloaded and visible in the message. Then enable the rule and manually run it on the message by selecting the message and pressing Option-Command-L. Doing this should provide adequate time for the Mail message to download before the rule is invoked on the message. This is not a solution to the problem, but will at least help you narrow down where the error is occurring.
Unfortunately for now there is no known fix, but one thing you can try is to remove Mail's preferences file in case the problem is rooted in an error in the file. To do this, close Mail down and then go to the user Library by holding the Option key followed by selecting Library from the Go menu. Then go to the preferences folder and remove the file called "com.apple.Mail.plist." When this is done, relaunch Mail, enter your account information again (your downloaded messages and mailbox organization should remain intact), set up your rules, and try testing them again.
If you are still not able to get attachments forwarded using rules, then you can also try setting the rule to redirect the incoming message instead of forwarding it. Unlike forwarding, which sets you as the sender, redirecting keeps the original sender as the reply-to address, but overall the result is the same in that the message gets automatically sent off to a secondary address.