It's amazing the number of times I've received email saying something like, "Here's the first eight digits of my credit card number. I'll send the second eight in another email (for security)."
This is not security. I wonder how people come up with these ideas. Do they think criminals accidentally intercept random emails sometimes? Or that they are able to intercept some messages, but there's no way they would be able to get two in a row?
If you're concerned about security, ask someone who is knowledgeable about the subject. In the case of email security, use PGP/GPG or S/MIME.
The state is that great fiction by which everyone tries to live at the expense of everyone else. - Frederic Bastiat