NDR = Non-Delivery Report
If I send you mail, but for whatever reason it doesn't get delivered, your server, or an intermediate server, may send an NDR. If this happens, I'll never know about it. My SpamBayes configuration now considers 100% of email sent by postmaster@ to be spam.
Mail from postmaster@ can now be broken down into four categories:
- Spam posing as NDRs to persuade users to read it, and to slip through anti-spam barriers;
- Viruses posing as NDRs to get you to open the attachments, and for the reasons above;
- Bounce messages from other domains where people have sent spam using your email address or, in my case, anything with my domain - my mail ISP, Demon, gives a child domain of demon.co.uk to each subscriber; I can use anything I like before the @;
- Genuine failures to deliver a message.
However, I don't actually send all that much email. I tend to receive notifications of comments to this blog, mailing list mail, and notification of replies to comments on web sites such as CodeProject. There's a little personal mail but not much.
I've recently become the Exchange administrator at work (an entirely new installation - we've only just got an Active Directory domain going and moved everyone to that). It doesn't look like you can configure it not to send a copy of the failed message back to the sender - should I configure it not to send NDRs at all?
However, if we did that, perhaps almighty fuck-ups like the one that happened last week - where we asked to begin the process of transferring a domain, and the ISP went ahead and did the whole transfer, rewriting the MX records to point to the Exchange server - wouldn't be detected as quickly.