To send a text message to a friend using your regular email account, you address it with the 10 digit phone number @ the email Web site URL of your friend's cell phone carrier. Note that's not the same as their main URL, it's a special one for text messaging. For example, AT&T is firstname.lastname@example.org, Verizon is email@example.com and T-Mobile is firstname.lastname@example.org. For other carriers, search "how to email text message to cell phone" on the support section of their Web site to find the text message email address.
Anyone can send you a text message from their cell phone to your email by entering your email address in the "To" field.