Blind Carbon Copy.
I know it seems sort of weird, but the To: line doesn't really tell the email server who to send it to. You use To:, CC:, and BCC: to tell your mail client who to send the message to. The mail client then tells the server what email addresses to send the message to. To: and CC: become lines in the message, but BCC: doesn't (because it's "Blind"). Those lines are actually a part of the message, rather than commands to tell the mail server how to deliver the message.
Depending on your mail server, your headers may show more information.
Code:
Return-path: <invisibill_AT_gmail.com>
Envelope-to: invisibill_AT_invisibill.net
Delivery-date: Wed, 13 Jul 2005 11:10:31 -0400
Received: from invisibi by eos.horsepower.site5.com with local-bsmtp (Exim 4.51)
id 1DsisZ-0004HY-NP
for invisibill_AT_invisibill.net; Wed, 13 Jul 2005 11:10:31 -0400
Received: from [64.233.170.194] (helo=rproxy.gmail.com)
by eos.horsepower.site5.com with esmtp (Exim 4.51)
id 1DsisZ-0004HU-KY
for invisibill_AT_invisibill.net; Wed, 13 Jul 2005 11:10:31 -0400
Received: by rproxy.gmail.com with SMTP id a41so224434rng
for <invisibill_AT_invisibill.net>; Wed, 13 Jul 2005 08:10:34 -0700 (PDT)
DomainKey-Signature: a=rsa-sha1; q=dns; c=nofws;
s=beta; d=gmail.com;
h=received:message-id:date:from:reply-to:subject:mime-version:content-type:content-transfer-encoding:content-disposition;
b=PTvtnyUI24J4bLAomX921r9BxO0FN+msSSfTDll2pdx2Q+c3MjxBfOWamiPVI/oxX/zLyYvvqeCeEvaEw8ceFniyrY4qB6b1ntp1kyF4gVriqc+qZlYn46AYyLWGBYYVmMRjhLMDW/EeKsoKlXufKSFeOHXmw0jI0MAK1TIt1JE=
Received: by 10.38.12.21 with SMTP id 21mr857120rnl;
Wed, 13 Jul 2005 08:10:34 -0700 (PDT)
Received: by 10.38.24.75 with HTTP; Wed, 13 Jul 2005 08:10:34 -0700 (PDT)
Message-ID: <544f2ba6050713081072a92d42@mail.gmail.com>
Date: Wed, 13 Jul 2005 11:10:34 -0400
From: Bill <invisibill_AT_gmail.com>
Reply-To: Bill <invisibill_AT_gmail.com>
Subject: Test
Mime-Version: 1.0
Content-Type: text/plain; charset=ISO-8859-1
Content-Transfer-Encoding: quoted-printable
Content-Disposition: inline
X-Spam-Checker-Version: SpamAssassin 3.0.4 (2005-06-05) on
eos.horsepower.site5.com
X-Spam-Level:
X-Spam-Status: No, score=-2.3 required=5.0 tests=AWL,BAYES_00,MISSING_HEADERS,
RCVD_BY_IP autolearn=ham version=3.0.4
Testing
That's an email I sent from my Gmail account to my main account with BCC. There's not even a To: line in it. The Envelope-to: header and the Received: headers show that it really was going to my address though.