Have you ever received an email message that appears to be addressed to someone else? Chances are very good that it wasn’t by mistake. Spammers often use the Bcc: (blind carbon copy) function of email when sending spam.
The screenshot below provides an example of an email using the Bcc: field.
All three of those email addresses will receive the email, but it will appear as though the message was sent to just the addresses in the To: field.
Except when it is mis-used by spammers, the Bcc: field actually helps to avoid spam. When sending a message to many people, it is considered good email etiquette to use Bcc:. This way when email is sent to many people, no one person will know other recipients’ email addresses. If the email falls into the hands of a spammer, the spammer will not be able to “harvest” all of the addresses.