This article was first written in June 2004 for the BeezNest technical
website (http://glasnost.beeznest.org/articles/131).
You should always avoid to send e-mails to many recipients at the same  time with their address in the 
To: or 
Cc: field.
The reason is that if you use 
Bcc:, any recipient will not be  able to see other's e-mail address.
Why would you want that?
Because many viruses on the Internet these days know how to go into your  e-mail archives and search for the e-mail addresses that you can see too.  If you use 
To: or 
Cc:, the Windows-running PCs of the  recipients become potential war-machines ready to attack any others'. If  you use 
Bcc:, the others' addresses won't appear into others'  e-mail, and you are safe to spam and viruses as well as the 
bad guy(tm).