<apex:commandButton value="Send" rendered="{!showEmail}"
     onClick="sendEmails();return false;"
     styleClass="slds-button slds-button_neutral" />

onClick="if(!confirm('Are you sure you want to send the email?'));return false;"

function sendEmails(){
   var temp = document.getElementsByClassName('emailbody')[0].id;
   var ckeditorBody = CKEDITOR.instances[temp].getData();
   CallSendEmail(ckeditorBody);
}

<apex:actionFunction name="CallSendEmail" action="{!sendEmail}" rerender="errorMessageId">
    <apex:param name="Richbody" value="" assignTo="{!body}"/>
</apex:actionFunction>