When you send an email to lots of people, and put all of their addresses either in the “To:” or “Cc:” field of your email client, you send everyone’s email address to everyone you send the email to.
This is at best bad manners, and at worst a serious breach of privacy – they may not want their email address sent to other people and potentially sent to more people they don’t know if/when the email is forwarded.
Fortunately there is an easy solution, use the “Bcc:” field:
So, if you’re sending an email to many recipients it is good “netiquette” to put one of your own email addresses in the “To:” field (so you’ll receive a copy, which you can delete if you wish) and put all of the other recipients’ email addresses in the “Bcc:” (Blind Copy) field. In this way you don’t expose someone’s email address to others, as they may not want their email address passed around!
Someone who agrees with me : to-bcc-or-not-bcc-email-etiquette.html