Most cellular carriers allow you to send an email to a cell phone number and have it appear as a text message (SMS message). For AT&T, for example, you send an email to email@example.com, obviously replacing the "1235551234" with the cellphone number you're sending to. On the receiver's phone, it appears as a text.
There are some tricks. For example, if the message length is very limited. It depends on the cell carrier, but 140 to 160 characters is about the max. You can run into problems if you violate this: for example, T-Mobile will silently drop emails that are longer.
Other problems can arise. For example, AT&T won't accept the email if there's no "From" address. I'm speaking from a coding perspective here (sending the emails from, say, PHP code), but I've found the following two things to be key:
1. Keep the email short. 100 characters is safe.
2. Always provide a "subject" line and "from" email address, even if older phones can't even see those fields or use them.