Emails can be send to several recipients at once by using the CC or BCC fields which send a Carbon Copy or Blind Carbon Copy to the recipients listed there. Using the BCC option is useful if you want that every recipient is only able to see his email address and not the email of everyone else who receives a message.
This is also a privacy issues and some will be really angry if you do not send mails to multiple recipients this way. It happens however that not all messages receive the recipient listed in the BCC field. This can be the case if you did not add a recipient in the TO field as well.
Some mail programs and filters block messages that have no TO recipient. The easiest workaround is to add your own email address to the to field which should take care of all the mail filters and programs that filter the mails otherwise.

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“Using the BCC option is useful if you want that every recipient is only able to see his email address and not the email of everyone else who receives a message.”
From my experience, if you get BCC’d on an email, you don’t see your own address. The BCC field doesn’t show up at all.
Ed you are right of course, BCC is not shown in the mail headers. You get some pointers though that make it easy to determine the account that it was send to.
Some like Gmail also use a header called Delivered-To which lists the email it was send to.