From an iPad, iMessage will only work between another iOS device or OS X computer. So if the recipient does not have an iPhone, iPad, iPod Touch or Mac computer (and must have it set up to use messages) then it will not work.
On an iPhone, you can still send a message as an SMS to another mobile phone (ie. a mobile phone other than an iPhone) as long as you have the option "Send as SMS" enabled. If the iPhone detects the "to:" address as registered to another iOS device, it will use Apple's services and send as an iMessage, otherwise it will send as SMS (if possible).