You've just discovered some amazing or humorous thing and you feel the urge to send it out to everyone you know. If you decide to send this item out via e-mail, don't put everybody in the "To:" or "CC:" fields, use "BCC:" instead.
The only exception to this rule is if you really need every recipient to see who else received that e-mail message. Most of the time this is not necessary and is not a good idea. The "BCC:" field stands for Blind Carbon Copy. E-mail addresses placed in that spot will be recipients of the message but the BCC: addresses won't be printed on each message. The identities of the recipients will be kept private.
There are four problems with putting every recipient's name in the "To:" field. Since they behave similarly, assume that this also applies to the "CC:" field.
The e-mail message is unnecessarily bloated with an arbitrarily large block of e-mail addresses. If you mail to 300 people, 300 e-mail addresses are sent with each and every e-mail. In some e-mail reading programs, this will require the recipient to have to page down a few times just to get to the contents of your e-mail.
Some of the recipients won't like to have their e-mail address made public. When your acquaintance Joe gives you his e-mail address, he assumes that you won't add it to some SPAM list. But by making his address public by sending it out to a large number of people mostly unknown to Joe, you are coming very close to that. Even though they aren't large scale spammers, some people are very enthusiastic about sending their humorous, political, religious, or whatever content to as many people as they can. Any e-mail address that passes to them is fair game.
What happens when one of these recipients uses the Reply feature on their e-mail program? Many of these programs will send the reply message to each and every recipient on that massive list. This is sometimes a hidden aspect. Joe replies to your mass mailing with some private comments to you. But Joe's message is not private. Joe is not paying close attention and it is sent to everyone on the list.
The worst outcome is a byproduct of the fact that these e-mail addresses are made public. There are worse things than enthusiastic mini-spammers. There are virus infections in many people's computers that cause virus containing mail to be sent out to everybody known to their e-mail program. Any e-mail address found in any e-mail message is fair game for the automatic e-mail sent by this type of virus. This type virus is only known to affect users of Microsoft Outlook, but this is a very popular e-mail program. Since any one of these mail recipients might be running an infected copy of Microsoft Outlook, by sending all of the e-mail addresses with each message, you risk turning all of these e-mail addresses into recipients of the virus.
The solution is simple. Respect the privacy of your mailing list recipients. Don't spread their e-mail addresses unnecessarily. Put the list of recipients in the "BCC:" field instead of the "To:" or "CC:" positions of your e-mail program. You can actually leave both of those places blank. The recipient will see something like "To: Undisclosed Recipients" in that case.