Why you shouldn't use BCC when you email groups of people.
* If I receive mail sent to "undisclosed-recipients", I assume the worst: that you are sending something to as many people as you can, and you don't want me to know that. I immediately think that whatever the correspondence is, it is for your benefit, not mine. This is also known as spam.
(note: this first point also applies when a mail is sent just to me, but the message is clearly a form-mail, e.g. it starts with "Dear ,"
* If you're inviting me to something via email and I see it is sent to "undisclosed-recipients", I assume the worst: that you are only inviting the lamest of people (plus me) and your event will ultimately suck. Not something I'm likely to attend.
* If you are making a legitimate announcement to a group of people, you should be sending this to a mailing list, not using BCC. When I receive you message, it will look like this: From: the secretary To: …