Using an email address as imessage contact name
iMessage is a service from Apple which works from iOS devices and Macs with OS X 10.8 and later only and is using the device's data connection rather than the regular phone connection.

Using the e-mail address with another device's text messaging system will most-likely result into an error telling you that the e-mail address entered is an invalid phone number.

Sending a message with ANY device as regular e-mail will result into your friend receiving the message as e-mail instead of an iMessage.