BCC stands for blind carbon copy or blind cutesy copy and any address in the BCC field is invisible to other recipients.
The converse is true and if BCC is not used then all of the recipients details are visible to everyone else on the email.
There are many views on the etiquette of using BCC, this answer is from the perspective of the Data Protection Act 1998 and the The Privacy and Electronic Communications (EC Directive) Regulations 2003.
These rules apply to marketing emails where the individual can be identified e.g. email@example.com, and those to sole traders and unincorporated bodies. In these circumstances unsolicited marketing emails are illegal.
Failing to use BCC means that there is a data breach and it may be worth considering the outcome when an NHS trust sent out a newsletter in this way read more….