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

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?

einpoklum

Posted 2014-09-16T17:31:20.903

Reputation: 5 032

Answers

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.

lbutlr

Posted 2014-09-16T17:31:20.903

Reputation: 99

I just sent a message with myaddr@somewhere.tld in BCC, and I got the message with the "for myaddr@somewhere.tld". So, that can't be it. (I used Microsoft's Office365's SMTP server.)

– einpoklum – 2014-09-16T17:56:32.333

That depends on your mailserver. The short version is, if you are not in the To or Cc header, you were BCCed. Mailinglist messages are technically BBCed to you. If your question is "can I tell the difference between a BCC and a mailinglist" then the answer is "maybe, but not for sure". – lbutlr – 2014-09-16T18:08:29.283

If it depends on my mailserver, then your answer is incorrect. "Maybe, but not for sure" is a different answer than what you wrote. – einpoklum – 2014-09-16T18:10:14.813

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.

Barmar

Posted 2014-09-16T17:31:20.903

Reputation: 1 913

How common are these 'no-trace' lists, or rather list servers, today? – einpoklum – 2014-09-16T21:13:22.490

They're probably very common for things like internal email lists within an enterprise. Probably not so common for public mailing lists like Yahoo Groups, because it's important to let people know how to contact the list admin, unsubscribe, etc. – Barmar – 2014-09-16T21:16:44.640