Why does iMessage randomly send as text instead?

Group iMessage with all recipients using the correct settings. Receive at/caller id both set to email address, send as sms set to off. One of the members of the group will randomly have his blue iMessage input box turn green to text message. When this happens, his phone will instead send the message from his @mms.att.net text address to our email inbox. All of us have each others phone/email saved properly in the contact information as well. The group message on his phone looks unaffected minus the change from blue/green between iMessage and text message, but on the other hand the rest of us are getting our inboxes flooded with all of this messages via email. With the group set up through all contacts email addresses, and send as sms set to off by each recipient, the iMessage should ideally never turn green to text at any point. This has been tested with all members on 3G, and even with all of us at home on Wi-Fi as well. The only problems I've been able to find from searching seem to be when people don't have all of their settings correct. I've used one situation from tonight as an example, but it has been known to happen here and there with no explanation. Has anyone else experienced this yet?