A fast tip: Many people send massive e-mails using standard e-mail software such as Outlook. This is a mistake, and not just because the time it takes, but also because the danger of sendig  the recipient’s list  if you aren’t careful with the fields that you use.

To avoid this, when we are  writing an e-mail message we can specify the recipients in any of the 3 following fields:

• To: field recipients are the audience of the message
• CC: (Carbon Copy) Recipients added to this field  are others whom the author wishes to inform of the message
• BCC: (Blinded Carbon Copy) Field recipients will discreetly  receive the e-mail and won’t see any of the other addresses.

So, if you send a massive mail using ‘To’ or ‘CC’, you will show all the adresses to all the recipients whom may consider you as a spammer. And this is not the worst: you may be going against the law, because you are publishing personal data.
 
Click on ‘To:’ or ‘CC:’ button to sort the addresses between the three fields:

 
You can also enable ‘BCC:’ field in Outlook 2007 by clicking on ‘Options’->’Show BCC’:

Anyway, the best choice for massive mails  is a specialized software as MAILCast, which will help you to manage, and hide ;-), all the addresses while saving up time composing and sending your e-mail.