Email Blind Carbon Copy

You can blind carbon copy (BCC) people on an email without the main addressees knowing about it. The story of the blind carbon copy function is another example of real-world processes imaging themselves in the virtual world.

The simple term "carbon copy" comes from the days of manual typewriters, when copies were made by typing on paper with several layers. Between each layer of paper was a thin sheet with carbon on the bottom side, so that when the typewriter keys hit the paper, the impact made a "carbon copy" of the letter on the paper sheet underneath. When you were finished typing a page, you would separate out the paper and throw away the carbon sheets, almost always getting some of the black carbon all over your hands.

You could get two, three, and four-layer carbon copy paper, and even some very thin six-layer paper. The top paper got the typewriter ink and looked the best. The top carbon copies always looked much crisper than the lower copies, as the force of the typewriter key strike dissipated through the layers. Occasionally, by accident or design, an extra carbon copy would be given to someone not on the official CC carbon copy address list. Because there was no way the official addressees could know about these extra copies, they were called "blind carbon-copies" (BCC).

It turned out that people liked this feature so much they built it into email. If you put addresses in the BCC field of an email it will be secretly copied to those addresses, and none of the other addresses in the To, CC, or BCC fields will know about it because the BCC field is not displayed on incoming messages.

Some of the ways that the BCC field can be used are listed below:

  • Plain old secrecy. You can blind copy an email to someone without the other addresses knowing that you did so to make a third party aware of an important issue, or to establish an independent confidential record of your email. However, this sort of thing is ethically problematic: you should have a good and ethical reason for not letting the primary addressee know that the email is being copied to someone else, since blind copying is basically a form of deception.
  • Copying yourself. You can blind copy yourself to a second email address, for example, to copy an important email from your home address to your work address, or vice versa.
  • Network diagnostics. You can double-check that an important email makes it out onto the Internet backbone by blind copying yourself on the email at another address, preferably at a different domain name. If the blind copied email to yourself arrives at your other address, then you can be reasonably sure that the copy of the email to the main addressee you are concerned about made it from your Internet service provider at least as far as the Internet backbone, which assures you that your local system, email server, and network connection are functioning well. If the intended email still doesn't arrive at its main destination, then the problem is likely with the addressee's network or email server.
  • Broadcasting. You can blind copy several email addresses at once in the BCC field if you are sending the email to more than one person. This feature of the blind copy is useful when for privacy reasons you don't necessarily want all of the recipients to know the addresses of the others, such as when emailing an invitation to several otherwise unrelated people, or if your email provider has problems and when they recover you need to send an email to all of your address book warning them that mail they sent in the past few days may have been lost.
  • Information. You can surreptitiously copy one friend on a joke email to another friend.

If you don't enter at least one valid email address in the To or CC field, then some email programs will indicate a notation like "Recipient list suppressed" in the To field, which tells the recipient at least that there are BCC addresses. If you don't want that to happen, you need to enter at least one valid email address in the To field, such as your own address, perhaps with a custom text description like the following:

To: "Gardening Friends" <>

Note there is a potential security flaw in the BCC feature. According to the conventions of the SMTP protocol, all addresses, including BCC addresses, are included in every email as it is sent over the Internet. The BCC addresses are stripped off blind copy email only at the destination email server. Therefore, if the addressee controls their email server or can access it, they could examine the BCC addresses on every email they receive. SMTP is designed this way for a couple of reasons:

  • Efficiency. It would take a lot more code and processing time to create a unique addressee list for each email to each BCC destination.
  • Efficiency again. With this convention, only one email needs to be sent to each domain. The email server at each domain reads all of the BCC addresses and sends a copy of the email (without any addresses) to each of the recipients at its domain.

Very occasionally, an email server will be misconfigured and not strip off the BCC list on email it sends to its local users, revealing the complete blind copy address field to users that receive the email at that domain. Therefore, BCC is very good but not perfect at keeping addresses confidential, and should not be relied on for the most critical and sensitive of communications. To avoid this problem you can always send the email to the main addressee, and then forward it old fashioned-way to those that you wish to have a copy of.

Source: William Stewart,
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Last modified: Wednesday, January 20, 2021, 3:18 PM