How can I send an iMessage to an e-mail address?
How can I send a message via e-mail to an e-mail address?
iPhone 6, iOS 8.3, pc with Outlook/iCloud
You can't typically. You CAN use email to reach many carriers via SMS (text) message but not the other way around. Typically, you can Google terms like "email to SMS verizon" to find out the correct format for the carrier of the person that you are trying to reach.
There may be third-party services that allow SMS to email this but it would be very atypical.
Why would you not just email the person?