Postfix appends a Delivered-To: header when a message is delivered to an address. This feature allows postfix to catch mail loops; however, the Delivered-To: header interferes with the operation of Majordomo. First, when using moderated lists, moderators must manually remove the Delivered-To: header before sending the approved message back to the list. (Otherwise Postfix will recognize that the approved message has already been sent to the list, and Postfix will stop the mail "loop.") Secondly, the Delivered-To: header reveals the secret address that delivers mail to everyone on a list; this behavior allows people on the list to discover this secret address allowing them to bypass the resend script.
The easiest solution to this problem is to avoid using moderated lists and accept that some people will be able to bypass resend. If this is unacceptable, better solutions are discussed in the Postfix FAQ.