Messages sending as iMessage to non iOS devices

You cannot because iMessage uses the internet to send to iOs Devices.

This seems to be a common problem. The only instructions I could find on the Apple web site state that iMessage users who switch to other devices should turn off iMessage before doing so. It sure would be nice if we senders had the ability to toggle iMessage on/off for each contact.

[extracted from iOS: Troubleshooting Messages]

f you plan on transferring your SIM card or phone number from an iPhone to a device that does not support iMessage, turn off iMessage in Settings > Messages first. If you do not, other iOS devices may continue to try to send a message using iMessage first, instead only using SMS or MMS.

The problem is that once a phone number has been verified to support iMessage the phone doesn't check again. You can try the following (I'm just guessing here):

1. Delete the contacts that had worked with iMessage that no longer have iPhones.
2. Reboot the phone by holding HOME and SLEEP until an Apple logo appears.
3. Add the contacts back.

Type out your message then after sending it, press and hold the message. A bubble will come out with "send as text message"...from then on, that session will be text msg instead of iMessage.

Looks like if after some minutes the iMessage is finally converted to a standard SMS when the receiver has no DATA. I suppose this is the SMS fallback feature.