How can I tell whether I was BCCed to a MIME message?

Question

In a MIME message, if my address is in the TO or CC fields, I know it was addressed to me in one of those ways. But what about BCC? Can I tell - from the copy of the message as I receive it from the mail server - that I was BCCed, rather than, say, having been subscribed to a mailing list to which the message was sent?

Answer 1

If you receive a message that was not addressed to you in the To or CC field and does not have your email address in the top Received: header then the message was delivered to you because of a Bcc.

The Received header will look something like this:

Received: from mail-in5.apple.com (mail-out5.apple.com [17.151.62.27])
    by mail.server.tld (Postfix) with ESMTPS id B2CCD50D4B3
    for <user@hostedomain.tld>; Tue, 16 Sep 2014 01:29:09 -0700 (PDT)

That 'for user' part will not be in a BCC message.

NB: A mailinglist is essentially a BCC, so it doesn't look all that different and your mailserver may not include that portion of the Received header. Also, a lot of mailing lists will include a custom Return-path that contains an encoded form of your email address to help with bounce processing.

Answer 2

There's no inherent way to tell the difference between receiving an email because of BCC or a mailing list. Some mailing list software will include information about the list somewhere, either in the headers or in the text itself; for instance, it might have a List-Unsubscribe: header. But many mailing lists are implemented simply using mailserver aliases that translate the list address into a list of member addresses; this type of mail distribution usually leaves no special traces anywhere in the message.