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.
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 – 2014-09-16T17:56:32.333That 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