--- a/mailnews/compose/test/unit/test_messageHeaders.js
+++ b/mailnews/compose/test/unit/test_messageHeaders.js
@@ -264,17 +264,17 @@ function* testNewsgroups() {
     .subscribeToNewsgroup("mozilla.test");
   let identity = getSmtpIdentity("from@tinderbox.invalid",
     getBasicSmtpServer());
   fields.newsgroups = "mozilla.test, mozilla.test.multimedia";
   fields.followupTo = "mozilla.test";
   yield richCreateMessage(fields, [], identity);
   checkDraftHeaders({
     // The identity should override the compose fields here.
-    "Newsgroups": "mozilla.test, mozilla.test.multimedia",
+    "Newsgroups": "mozilla.test,mozilla.test.multimedia",
     "Followup-To": "mozilla.test",
     "X-Mozilla-News-Host": "localhost",
   });
 }
 
 function* testSendHeaders() {
   let fields = new CompFields();
   let identity = getSmtpIdentity("from@tinderbox.invalid",

--- a/mailnews/mime/src/extraMimeParsers.jsm
+++ b/mailnews/mime/src/extraMimeParsers.jsm
@@ -12,17 +12,17 @@ function parseNewsgroups(headers) {
 
 function emitNewsgroups(groups) {
   // Don't encode the newsgroups names in RFC 2047...
   if (groups.length == 1)
     this.addText(groups[0], false);
   else {
     this.addText(groups[0], false);
     for (let i = 1; i < groups.length; i++) {
-      this.addText(", ", true);
+      this.addText(",", false); // only comma, no space!
       this.addText(groups[i], false);
     }
   }
 }
 
 jsmime.headerparser.addStructuredDecoder("Newsgroups", parseNewsgroups);
 jsmime.headerparser.addStructuredDecoder("Followup-To", parseNewsgroups);
 jsmime.headeremitter.addStructuredEncoder("Newsgroups", emitNewsgroups);