KB article 812806 describes a serious bug in Exchange 2000 and Exchange 2003, as well as its fix. The bug manifests itself when an Exchange user sends an SMTP message to an external mailing list address, and the delivery attempt to one or more members of the mailing list generates an NDR, the NDR presented by Exchange to the original sender makes it appear that delivery to the list address itself failed.
Well, this bug has resurfaced in Exchange 2007. A Microsoft PSS support engineer has confirmed to me that the fix described in the aforementioned KB article does not work with Exchange 2007, nor is there a fix in place. An ECR was apparently filed last year to address the issue, but it seems that there has been no progress made on getting a fix out.
Can't Micosoft fix a bug and keep it fixed? Especially on such a blatantly serious bug?
Update: Within minutes of posting this, I received an update from PSS. A hotfix is in the works. Release is tentatively slated for late April.