ItemSend

The on-send API requires VersionOverrides v1_1.

link - github.com/OfficeDev/Outlook-Add-in-On-Send 

<Hosts> 
    <Host xsi:type="MailHost"> 
        <DesktopFormFactor> 
            <!-- The functionfile and function name to call on message send.  --> 
            <!-- In this case the function validateSubjectAndCC will be called within the JavaScript code referenced in residUILessFunctionFileUrl. --> 
            <FunctionFile resid="residUILessFunctionFileUrl" /> 
            <ExtensionPoint xsi:type="Events"> 
                <Event Type="ItemSend" FunctionExecution="synchronous" FunctionName="validateSubjectAndCC" /> 
            </ExtensionPoint> 
        </DesktopFormFactor> 
    </Host> 
</Hosts>