fix: make sure references header only contains strings

Fixes https://github.com/Salesflare/Server/issues/7122

lib/utils.js

@@ -1,5 +1,6 @@
 'use strict';
 
+const _ = require('lodash');
 const MailComposer = require('nodemailer/lib/mail-composer');
 const SimpleParser = require('mailparser').simpleParser;
 
@@ -71,6 +72,15 @@ exports.parseRawMail = (rawMessage, callback) => {
 
                     return v;
                 });
+
+                // We want to make sure the references headers only contains strings, other stuff can lead to problems downstream
+                // See https://github.com/Salesflare/Server/issues/7122
+                if (key === 'references') {
+                    parsedMail.headers[key] = parsedMail.headers[key].filter((reference) => {
+
+                        return _.isString(reference);
+                    });
+                }
             });
         }