const DEFAULTS = {

  internalDomains: ["yourcompany.com"],

  recipientThreshold: 50,

};

function getConfig() {

  // … retrieve the internalDomains and recipientThreshold

  //   settings from the Office “roaming settings” or

  //   use the DEFAULTs if not available …

}

function isInternal(emailAddress, internalDomains) {

  // return true if the email address is from one of the internal domains

}

function getRecipients(field) {

  // field is “to”, “cc” or “bcc”

  // use getAsync() to retrieve the content of that field

}

async function onMessageSend(event) {

 try {

  const config = getConfig() ;

  const email = Office.context.mailbox.item ;

   const [toRecipients, ccRecipients] = await(Promise.all([

    getRecipients(item.to), getRecipients(item.cc)]) ;

  const allRecipients = toRecipients.concat(ccRecipients);

  const externalRecipients = allRecepients.filter(

    (r) -> !isInternal(r.emailAddress, config.internalDomains));

  if (exteralRecipients.length > config.recipientThreshold) {

    const userMessage = “... warning message for user ...” ;

    event.completed({

      allowEvent: false,

      errorMessage: message,

      sendModeOverride: Office.MailboxEnums.SendModeOverride.PromptUser,

    }) ;

  } else {

    Event.completed({allowEvent: true});

  }

 } catch(error) {

   console.log(“error message”) ;

   event.completed({allowEvent: true});

 }

}