Email is inherently unsafe.

I might have been misleading by saying "secure channel". Email is unsafe by its nature for two reasons (at least for now):

1) Your email is like "postcard". No rule exists to enforce emails to be encrypted during transmission from sender's server to recipient's server, or to be encrypted when stored on the email server. System administrators of your email server or firewall server can see your emails, so do hakers if the server is compromised.

2) The "broadcast" nature of network packets. Network tapping requires no special equipment and is much easier than wire tapping.

So never use email to send private information such as password, credit card number, social security number, and intimate pictures.