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Few things are as irritating as receiving a message your e-mail address listed along with others for people you don't know. Not only is this poor etiquette, but it also invites misuses of those addresses by any of the recipients or anyone they might forward the message to.
When people provide you with their e-mail address — either directly or by sending you a message — there is the implicit expectation that you will not reveal it to others without their permission.
Just as you wouldn't appreciate your place of employment giving out personal contact information, many people feel the same way about their e-mail address.
In fact, privacy has become a serious issue on the Internet and concern will continue to grow as people realize the commercial value of their private information and as they fight a losing battle against spam (unsolicited and unwanted commercial or non-commercial e-mail).