As long as you know the SMS e-mail gateway for the carrier of the recipient, yes you can. For example sprint is phone number @messaging.sprintpcs.com. So if you want to send an e-mail to the phone number 1234567890, you'd e-mail
1234567890@messaging.sprintpcs.com. Other carriers will have different formats, but as long as you know it you should be able to send an e-mail to the SMS gateway on any carrier.