When you send to Bcc: addresses you create two separate messages. One goes to the To: and Cc: addresses, one goes to the Bcc: addresses only. You are sending to the SMTP mail addresses using a RCPT TO: SMTP address. You need to run session logging to see exactly what you are sending to the SMTP host. Here's a sample of a session log. Note the e-mail addresses in the body of the message sent after the DATA line has nothing to do with the actual SMTP addresses in the MAIL FROM: and RCPT TO: lines.
--- Sat, 20 Jun 1998 15:04:12 ---
Connect to 'scruznet.com', timeout 90.
15:04:13 >> 220 scruz.net ESMTP Sendmail 8.8.5/1.34 ready at Sat, 20 Jun 1998 15:04:14 -0700 (PDT)\0D\0A
15:04:13 << HELO thomas.lmms.lmco.com\0D\0A
15:04:13 >> 250 scruz.net Hello stephens.sj.scruznet.com [165.227.102.91], pleased to meet you\0D\0A
15:04:13 <<
MAIL FROM:<stephens@scruznet.com>\0D\0A
15:04:14 >> 250 <stephens@scruznet.com>... Sender ok\0D\0A
15:04:14 <<
RCPT TO:<stephens@scruznet.com>\0D\0A
15:04:14 >> 250 <stephens@scruznet.com>... Recipient ok\0D\0A
15:04:14 <<
DATA\0D\0A15:04:14 >> 354 Enter mail, end with "." on a line by itself\0D\0A
15:04:14 <<
From: "Thomas Stephenson" <support@tstephenson.com>\0D\0A15:04:14 <<
To: test@tstephenson.com\0D\0A15:04:14 << Date: Sat, 20 Jun 1998 15:04:12 -0700\0D\0A
15:04:14 << MIME-Version: 1.0\0D\0A
15:04:14 << Content-type: text/plain; charset=US-ASCII\0D\0A
15:04:14 << Content-transfer-encoding: 7BIT\0D\0A
15:04:14 << Subject: test\0D\0A
15:04:14 << Priority: normal\0D\0A
15:04:14 << X-mailer: Pegasus Mail for Win32 (v3.01b)\0D\0A
15:04:14 << \0D\0A
15:04:14 << test message body\0D\0A
15:04:14 << \0D\0A
15:04:14 << .\0D\0A
15:04:15 >> 250 PAA08117 Message accepted for delivery\0D\0A
15:04:15 << QUIT\0D\0A
15:04:15 >> 221 scruz.net closing connection\0D\0A
--- Connection closed normally at Sat, 20 Jun 1998 15:04:15. ---