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BCC Mode

"BCC Mode" is the ability of an email program to address a single outgoing email to multiple recipients.  The email program only sends the email once.  The mail server then delivers it to each recipient.  This greatly speeds up large mailing operations because the program only needs to send a fraction of the number of emails.

Note:  Certain other mail programs refer to this feature differently.  Gammadyne Mailer refers to it as the Identical Send Mode (located at the top of the "Send" branch).

How It Works

Following is a sample conversation with an SMTP server.  "S:" preceeds data sent from the mailing program, "R:" preceeds the data that is sent back from the server.  Note the repetition of the "RCPT TO:" command.  There is one "RCPT TO:" command per recipient.  The body of the email is sent, just once, following the "DATA" command.

R: 250 MIT-Multics.ARPA

R: 250 OK

S: RCPT TO:<Jones@MIT-Multics.ARPA>
R: 250 OK

S: RCPT TO:<Green@MIT-Multics.ARPA>
R: 550 No such user here

R: 250 OK

R: 354 Start mail input; end with <CRLF>.<CRLF>

S: From: Smith@ISI-VAXA.ARPA
S: To: Jones@MIT-Multics.ARPA
S: CC: Green@MIT-Multics.ARPA
S: Subject: Hello
S: Date: 22 Sep 2001 02:31:06 -0500
S: MIME-Version: 1.0
S: Content-Type: text/plain
S: This is the body of an email.
S: .
R: 250 OK

R: 221 BBN-UNIX.ARPA Service closing transmission channel


  • The use of BCC Send Mode can greatly increase the speed of a mailing.  The body of the email is only uploaded once per batch of recipients, which is typically 20 to 60 in number.


  • There is a risk of overwhelming the mail server.  The email program is addressing each email to a large number of recipients.  The mail server then has to send a separate message to each recipient.  This makes it relatively easy to place great load on any mail server.
  • Every mail server has a limit on the number of recipients that a single email can be addressed to.  Unfortunately, ISP's do not usually publicize the limit.  The only way to determine the exact limit is to experiment.  Once the limit is reached, the mail server will start producing errors.  The limit is typically 50-100.
  • The messages can't be personalized - all recipients will get an identical copy of the email.  This even includes the "To:" header field.
  • Because each recipient in a batch receives a message with the same Message-ID header, many mail servers will treat the messages as spam.
  • Direct Delivery is not compatible with the BCC Mode.