🐛 Fixed missing text version in bulk email (#11919)

closes #11917

- Pass text-only version to mailgun as `text` not `plaintext`
- This ensures we send a text-only version of the email, and this in turn should help to improve spam scores

core/server/services/bulk-email/index.js

@@ -98,6 +98,10 @@ module.exports = {
 
             const messageData = Object.assign({}, message, batchData);
 
+            // Rename plaintext field to text for Mailgun
+            messageData.text = messageData.plaintext;
+            delete messageData.plaintext;
+
             return new Promise((resolve) => {
                 mailgunInstance.messages().send(messageData, (error, body) => {
                     if (error) {