Undisclosed Recipients

"When sending a mass mail to a list of recipients using a normal mail client you don't want to include all the recipients in the To: or Cc: fields, both because it leaks the addresses of all the recipients and because of the risk of reply allpocalypse. That's when a recipient does a reply-all in response to the mail, typically asking to be removed, to all the recpients, and people respond to that and millions of emails later everyone's mailbox is unusable."

"a Cisco employee sent an email to a "sep_training1" mailing list containing 23,570 members requesting that an online training be performed. The resulting storm of more than four million reply emails, many of which were requests to unsubscribe and facepalm images, generated over 375 GB of network traffic and an estimated $600,000 of lost productivity. The following month, a nearly identical email storm occurred when an employee sent a message to a Cisco group containing 34,562 members."

"So long-standing advice is to put all the recipients of a bulk mail into the Bcc: field of the email. That way none of the email addresses will appear in the email, folks won't be able to reply-all, there's no leak of addresses and it's all good. Except… that means there'll be no email addresses in the To: (or Cc:) headers at all."

Undisclosed recipients: is used in the To: header when a sender hides all actual recipient addresses by placing them in the Bcc: field. Bcc prevents exposure of recipients' addresses and blocks recipients from accidentally triggering reply-all storms. Large reply-all storms can generate millions of emails, hundreds of gigabytes of traffic, and significant productivity costs, as shown by corporate incidents involving mailing lists with tens of thousands of members. Because an email with no To: or Cc: recipients is syntactically problematic and can break some mail clients, mail systems insert a syntactically valid placeholder into the To: header instead of leaving it empty.

Read at Wordtothewise