Using BCC (Blind Carbon Copy)
When other individuals provide you with their email 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 individuals feel the same way about their email address. In fact, privacy has become a serious issue on the Internet and this concern will continue to grow as people realise the commercial value of their private information and as they fight a losing battle against spam (unsolicited and unwanted commercial or non-commercial email).
When you place email addresses in the 'BCC:' field of a message, those addresses are invisible to the recipients of the email. Conversely, any email addresses that you place in the 'To:' field or the 'CC:' field are visible to everyone who receives the message.
Keep in mind that many indivudials do not want their email address to be distributed to people they do not know (or even those that they do!) and that without explicit authority such distribution of an individual's email address is a breach of their privacy and contravenes the Data Protection Act 1998.
Using the 'BCC:' field to conceal email addresses acts as an anti-spam, anti-virus measure. If you use the 'To:' field or the 'CC:' field to send a message to a large group, it becomes more likely that the recipients will receive unwanted email. This could occur because recipients use the "Reply to All" feature to respond to your message. A second scenario that could create unwanted email involves spam programs that are designed to send spam to addresses found in the 'To:' and 'CC:' fields of messages. The 'BCC:' field protects against these scenarios.