When you send an e-mail to a large number of people, each recipient can see all of the addresses that the e-mail was sent to. This might not be a problem if it's just your family or close friends, but if you send to a large group of people, or some of the people you send to end up forwarding the e-mail to other people, the entire list may end up in the hands of an "e-mail harvester".
E-mail Harvesting is the act of collecting e-mail addresses that were not specifically given to you. E-mail addresses can be harvested from web pages, e-mails, directory services, marketing materials, business cards, etc. Many people do this semi-innocently, and send out nothing more than annoying forwards of jokes and hoaxes, but some people actually do it for profit. They collect e-mail addresses to sell to spammers, who send out junk e-mail.
Fortunately, there is something you can do to protect your friends and family from e-mail harvesters. It's called a "Blind Carbon Copy (BCC)". Here's how it works:
When you send an e-mail to someone, you have three places that you can enter a recipient's name / address. The TO field, the CC field, and the BCC field. The TO field is intended to hold the actual person(s) for whom the e-mail was written. The CC field, (known "Carbon Copy" or "Courtesy Copy"), is for people that the e-mail is not directed to, but need to be aware of its content. The BCC field is just like the CC field, but with one important difference - the names and e-mail addresses of those in the BCC field are hidden from all other recipients of the e-mail.
The BCC field can be used to send an e-mail to a large number of people, without revealing everyone's e-mail address to everyone else. This way, you protect the e-mail addresses from e-mail harvesters.
Putting everyone's e-mail address in the BCC field will usually work, but the e-mail looks very strange when received, because there is nothing in the TO field. It looks at first glance, that the e-mail was sent to no one, or that it was only sent to one person, with their name put in the wrong place. An easy way to get around this is to create an entry in your address book called "Multiple Recipients" (or something similar). Set the e-mail address for this entry to be YOUR e-mail address. Then, whenever you want to send out an e-mail to a large group of people, you can send it to "Multiple Recipients" and BCC everyone on your list. The e-mail they will get looks like this:
From: you@youraddress
To: Multiple Recipients
CC:
BCC: <each person sees their own e-mail address here>
Now you have protected everyone's e-mail address, but it is still clear to each recipient that he/she is one of many people who got the e-mail. Anyone who forwards this e-mail will not be passing along any addresses except for the original senders (you) and their own.