When you put a list of e-mail recipients in the "To" or "CC" line of an e-mail, every recipient can see that entire list. This can result in a privacy breach or simply cause a headache as multiple people "Reply All" and create a noisy e-mail thread. If you'd like to send out a mass e-mail without allowing the recipients to see your mailing list, you can use your e-mail program's BCC, or blind carbon copy, functionality. When someone receives a BCCed e-mail, he only sees his own e-mail address as the recipient, even if you sent that e-mail to thousands.
Click "Add BCC" underneath the "To" box in Gmail. Type or paste your comma-separated mailing list in the BCC box.
Click "Fields" > "Show BCC" under the "Options" tab in the Microsoft Outlook ribbon while composing a new e-mail. A BCC box will appear. Enter your mailing list into this box.
Select "View" --> "BCC Header" in Apple Mail to reveal the BCC box. Paste your mailing list e-mails into this box.
You do not need to list any e-mail addresses in the To or CC fields.
Only e-mail addresses listed in the BCC field are hidden. All BCCed recipients can still see any addresses you include in the To or CC areas.