If I've understood it correctly, you're building not an email client but rather an application with email capabilities. In that case, you don't need a
Cc: field because it's semantically unnecessary here. Per section 3.6.3 "Destination address fields" of RFC 2822 (Internet Message Format) the purpose of the
[to contain] the addresses of others who are to receive the message, though the content of the message may not be directed at them
while the purpose of the
To: field is
[to contain] the address(es) of the primary recipient(s) of the message.
According to these definitions, people are considered to be primary recipients if they are supposed to receive notifications, reports, or any other messages your application generates. Otherwise, you might be letting your users send semantically unrelated/unwanted messages (aka spam). Therefore, only one option (
To:) is necessary, and it's easily implemented with just one checkbox.
If you ever decide to include another recipient option, such as
Bcc:, you should go with the canonical control element for mutually exclusive options, which is the radio button. Checkboxes are used either for selecting multiple options or to record a Boolean state.
And if anyone raises the concern that radio buttons are confusing or visually unappealing for such a scenario (selecting email recipients & method of delivery), test the workflow with real users. The question to be asked during the tests must be "what they find to be the most confusing/annoying/surprising interaction" instead of "whether radio buttons confuse them". Beware, if the test results show that users do indeed prefer checkboxes in this situation, you'll need to solve yet another problem: what to do when all checkboxes in a row are marked. Hopefully, it never comes to it.
Nonetheless, you can improve the readability of the list as well as minimize wrong selections by adding zebra stripes and highlighting the selected rows. (The article on zebra stripes is about implementing them in web apps but the best practices on colors apply everywhere.)