When you need to send lots of people the same message, it may seem like a good idea to put all of the email addresses in the "To" field, and click send. I have received many emails like this, and often contact the sender to ask them not to do this. Here is an example of why it is not a good idea. This by-the-way is real; this actually happened...
When a computer is infected with a virus, one of the aims of that virus may be email address collection. That is, the sole purpose is to find all email addresses listed in the emails on that computer. Each email is searched, including all of the other recipients of each email.
This email address list is then sent back to the virus writer as a valid list of email addresses.
At this point I guess you can see what will happen next. The "Blue Pill" salesman will pay for access to that email address list. Everyone on that list will then get additional un-wanted mail.
So why am I writing this? Simple, there is a minor change in behaviour that can help to reduce the email addresses which are collected. Use the BCC field instead.
Scenario 1. Mr Smith sends 50 people an email with all of the addresses in the "To" field. All 50 of those machines now contain all 51 email addresses (50 recipients plus the sender). If any of those computers become infected, all will be sent junk.
Scenario 2. Mr Jones sends 50 people an email with all of the addresses in the BCC field. Each of the 50 machines now only contain 2 email addresses; the one it was sent from, and the single BCC email address that is visible to that computer.
The BCC Field does exactly what it says on the tin. Blind Carbon Copy. That means each addressee only sees their own email address.
Some email programs hide the BCC field by default, and you will need to click something to be able to see it. I hope you now understand why it is worth finding...
I mentioned above that this really happens, and it did in 2009. Someone I met at a networking event sent an email to everyone she had an email address for, including me. I asked her not to do this, and I was surprised more by the arrogance than the lack of knowledge. She sent more email over the next few weeks, and I asked her to remove my email address from her list, which she did. Two weeks later, another of the recipients' computers caught a virus. In the days that followed, I received email for all sorts of pills and watches. All from one of her contacts that I have never met. Fortunately in this case, when the infected machine was cleaned, the emails stopped. Often however once you are on a list, you will get spam forever and a day.