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If I want to send an email to multiple recipients without them knowing each other, can I consider the BCC field safe enough to do this?

Obviously the server will still know and have a log of who the recipients are, but if we assume that server will not be hacked would the BCC field be enough?

  • Seeing as your question was "without THEM knowing each other" , I would say yes, it would be enough. – katrix Nov 22 '16 at 19:32
  • The catch is that, and it happens, when a bcc member replies to all. – George Nov 23 '16 at 3:26
  • @George Doesn't that just reply to the sender, and all "known" receivers? – Cowthulhu Jan 9 '18 at 19:02
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The idea of the BCC field is indeed to keep recipients undisclosed from each other. But it should not be understood as a reliable security feature and I wouldn't trust it in a life-and-death situation.

Servers often add additional headers to emails during processing. These headers might, as a side effect, inadvertently reveal BCC recipients to everyone else. For example, this has been a problem with various email encryption implementations. Some research on that can be found in the paper Correcting Privacy Violations in Blind-Carbon-Copy (BCC) Encrypted Email. From the abstract:

We show that many widely deployed email encryption systems reveal the identities of Blind- Carbon-Copy (BCC) recipients. For example, encrypted email sent using Microsoft Outlook completely exposes the identity of every BCC recipient. Additionally, several implementations of PGP expose the full name and email address of BCC recipients.

I could imagine that similar problems exist with anti-spam filters or other forms of email processing that your mail server might employ.

Also, there have been flawed implementations in the past. For example, Microsoft Outlook Express Information Disclosure Vulnerability (CVE-2004-2137):

Microsoft Outlook Express 6.0 SP1 and prior contain a vulnerability that could cause private information to be disclosed to e-mail recipients. The application may arbitrarily allow a recipient to view the addresses listed in the BCC: field.

As an alternative I'd suggest you to send the mail to each BCC recipient individually. If there are many of them, you can use a bulk email plugin for your mail client that will send the mails one by one.

You can find a related answer here.

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