This is a user setting rather than a standard system setting, via menu path [Administration \ User preferences] you can specify which field the recipients populate on an outgoing email.

You need to choose the option Change the appearance of contacts email addresses in outgoing emails by click on the button [To / Bcc...]

This gives you the following 2 options

Use ‘To’ field

Put all the Recipients in the ‘To’ field. Every Recipient will be able to see the addresses of all the other recipients

Use ‘Bcc’ field

Put all the Recipients in the ‘Bcc’ list. No Recipient will see who else this e-mail was sent to.