If you receive an e-mail that has been sent to others, before you answer, think about whether you want only the sender to get your answer or if everyone needs to read it.
Often, business e-mails go to many people using TO: for one person and CC: for everyone else. When sending out a group e-mail, using BCC: often is preferable. With BCC: only the sender knows who got the e-mail; each recipient knows others were also on the distribution list, but they don’t see anyone else’s address.
I write a weekly business writing tip to hundreds of my corporate contacts and always send it BCC: because I feel it would be wrong of me to disclose all the e-mail addresses to everyone on my distribution list; it’s a question of their privacy. Also, recipients frequently write back to me, and I know there is no danger of them clicking REPLY ALL and annoying everyone else on the list.