This is the html for the Send button in Hotmail:
<a id="SendMessage" onclick="if(window.ComposeContactPicker)return Control.invokeStatic('ComposeContactPicker', 'sendMessagePrep', event, null);" href="java script:;" title="Send"><span class="Label">Send</span></a>
It's also important to know that this element exists within an iframe. This code works for me:
#include <IE.au3>
$oIE = _IEAttach ("Windows Live Hotmail")
If @ERROR Then
MsgBox(0,"","Failed to attach IE instance")
EndIf
$oIFRAME = _IEFrameGetObjByName ($oIE, "UIFrame")
$oA = _IEGetObjById ($oIFRAME, "SendMessage")
If @ERROR Then
MsgBox(0,"","Send button not present")
Else
MsgBox(0,"","Sending now")
_IEAction ($oA, "click")
EndIf