I want to send a mass email to some of my customers. I don't want them to see each others names/etc. How do I block the email addresses from showing up in the "To:" line?
You can do it so that the Bcc line will always show (it is hidden by default).
If you use MS Outlook 2003 then see this.
If you use MS Outlook 2007 then open a new message (like you are going to write a new email) then go to the options tab and click the big button that says "show Bcc".