You can read the following section to learn how ATT send text to email. If you can send a picture, a video, or a text message, write the email first.
AT&T needs the recipient's wireless number as well as @mms.att.net before it can send a picture or video message.
Text messages are sent by sending an email that includes the recipient's wireless number and their AT&T Text account.
Emails are sent by ATT instead of text messages. The email can be sent in the same way.