[The following is excerpted from
http://www.internetsecurityguide.com/websecurity/benefitsofbcc.shtml ]
Benefits of BCC
BCC, which stands for blind carbon copy, allows you to hide recipients in email messages. Unlike addresses in the To: field or the CC: (carbon copy) field, addresses in the BCC: field cannot be seen by other users.
Why would you want to use BCC?
There are a few main reasons for using BCC:
Many people frequently forward messages to their entire address books using CC. Encourage people who forward messages to you to use BCC so that your email address is less likely to appear in other people's inboxes and be susceptible to being harvested. To avoid becoming part of the problem, in addition to using BCC if you forward messages, take time to remove all existing email addresses within the message. The additional benefit is that the people you're sending the message to will appreciate not having to scroll through large sections of irrelevant information to get to the actual message.
How do you BCC an email message?
Most email clients have the option to BCC listed a few lines below the To: field. However, sometimes it is a separate option that is not listed by default. If you cannot locate it, check the help menu or the software's documentation.
If you want to BCC all recipients and your email client will not send a message without something in the To: field, consider using your own email address in that field. In addition to hiding the identity of other recipients, this option will enable you to confirm that the message was sent successfully.
Benefits | Importance | Where is BCC Field? | Macworld Magazine Discussion