diff --git a/mailnews/compose/src/MimeEncoder.jsm b/mailnews/compose/src/MimeEncoder.jsm
index 839a42d2fb..18dcfbce7f 100644
--- a/mailnews/compose/src/MimeEncoder.jsm
+++ b/mailnews/compose/src/MimeEncoder.jsm
@@ -94,7 +94,17 @@ class MimeEncoder {
         (strictlyMime && this._unPrintableCount) ||
         this._nullCount
       ) {
-        encodeP = true;
+        if (
+          this._isMainBody &&
+          this._contentType == "text/plain" &&
+          // From rfc3676#section-4.2, Quoted-Printable encoding SHOULD NOT be
+          // used with Format=Flowed unless absolutely necessary.
+          Services.prefs.getBoolPref("mailnews.send_plaintext_flowed")
+        ) {
+          needsB64 = true;
+        } else {
+          encodeP = true;
+        }
       }
 
       // MIME requires a special case that these types never be encoded.
@@ -408,12 +418,7 @@ class MimeEncoder {
 
       if (currentColumn >= 73) {
         // Soft line break for readability
-        if (i + 1 < this._bodySize && this._body[i + 1] == " ") {
-          out += "=20\r\n";
-          i++;
-        } else {
-          out += "=\r\n";
-        }
+        out += "=\r\n";
         white = false;
         currentColumn = 0;
       }
diff --git a/mailnews/compose/test/unit/test_messageBody.js b/mailnews/compose/test/unit/test_messageBody.js
index 708ca0f805..07ce6d279b 100644
--- a/mailnews/compose/test/unit/test_messageBody.js
+++ b/mailnews/compose/test/unit/test_messageBody.js
@@ -62,13 +62,12 @@ add_task(async function testQP() {
     "QP for non-ascii should work"
   );
 
-  // Bug 1689804 - Avoid a QP soft line break before a space.
+  // Test leading space is preserved.
 
   fields = new CompFields();
   fields.forceMsgEncoding = true;
   fields.to = "Nobody <nobody@tinderbox.invalid>";
-  fields.subject =
-    "Bug 1689804 - Save a space to the previous line on a quoted printable soft line break.";
+  fields.subject = "Leading space is valid in a quoted printable message";
   fields.body = "123456789" + " 123456789".repeat(6) + "1234 56789";
   await richCreateMessage(fields, [], identity);
 
@@ -81,7 +80,49 @@ add_task(async function testQP() {
 
   Assert.equal(
     body.trimRight("\r\n"),
-    "123456789 123456789 123456789 123456789 123456789 123456789 1234567891234=20\r\n56789"
+    "123456789 123456789 123456789 123456789 123456789 123456789 1234567891234=\r\n 56789"
+  );
+
+  Services.prefs.clearUserPref("mail.strictly_mime");
+});
+
+/**
+ * Test QP is not used together with format=flowed.
+ */
+add_task(async function testNoQPWithFormatFlowed() {
+  if (!Services.prefs.getBoolPref("mailnews.send.jsmodule")) {
+    return;
+  }
+  // Together with fields.forceMsgEncoding, force quote-printable encoding.
+  Services.prefs.setBoolPref("mail.strictly_mime", true);
+
+  let identity = getSmtpIdentity(
+    "from@tinderbox.invalid",
+    getBasicSmtpServer()
+  );
+  let fields = Cc[
+    "@mozilla.org/messengercompose/composefields;1"
+  ].createInstance(Ci.nsIMsgCompFields);
+  fields.forceMsgEncoding = true;
+  fields.forcePlainText = true;
+  fields.to = "Nobody <nobody@tinderbox.invalid>";
+  fields.subject = "Test QP encoding for trailing whitespace";
+  fields.body = "A line with trailing whitespace\t ";
+  await richCreateMessage(fields, [], identity);
+
+  let msgData = mailTestUtils.loadMessageToString(
+    gDraftFolder,
+    mailTestUtils.firstMsgHdr(gDraftFolder)
+  );
+  Assert.ok(
+    msgData.includes(
+      "Content-Type: text/plain; charset=UTF-8; format=flowed\r\nContent-Transfer-Encoding: base64"
+    ),
+    "format=flowed should be used"
+  );
+  Assert.ok(
+    !msgData.includes("quoted-printable"),
+    "quoted-printable should not be used"
   );
 
   Services.prefs.clearUserPref("mail.strictly_mime");
diff --git a/mailnews/mime/src/mimeenc.cpp b/mailnews/mime/src/mimeenc.cpp
index e2f72a5bec..1c9df3794b 100644
--- a/mailnews/mime/src/mimeenc.cpp
+++ b/mailnews/mime/src/mimeenc.cpp
@@ -966,11 +966,6 @@ nsresult QPEncoder::Write(const char* buffer, int32_t size) {
     if (mCurrentColumn >= 73)  // Soft line break for readability
     {
       *out++ = '=';
-      if (in + 1 < end && in[1] == ' ') {
-        in++;
-        *out++ = '2';
-        *out++ = '0';
-      }
       *out++ = '\r';
       *out++ = '\n';
 
diff --git a/mailnews/mime/src/mimemsg.cpp b/mailnews/mime/src/mimemsg.cpp
index 7efc659440..fcd86651f7 100644
--- a/mailnews/mime/src/mimemsg.cpp
+++ b/mailnews/mime/src/mimemsg.cpp
@@ -148,15 +148,6 @@ static int MimeMessage_parse_line(const char* aLine, int32_t aLength,
      */
     nl = (length > 0 && (line[length - 1] == '\r' || line[length - 1] == '\n'));
 
-    if (!mime_typep(kid, (MimeObjectClass*)&mimeMessageClass) && obj->options &&
-        !obj->options->is_multipart_msg && !obj->options->decrypt_p &&
-        mime_typep(kid, (MimeObjectClass*)&mimeInlineTextPlainFlowedClass)) {
-      // Remove any stuffed space.
-      if (length > 0 && ' ' == *line) {
-        line++;
-        length--;
-      }
-    }
 #ifdef MIME_DRAFTS
     if (!mime_typep(kid, (MimeObjectClass*)&mimeMessageClass) && obj->options &&
         obj->options->decompose_file_p && !obj->options->is_multipart_msg &&
diff --git a/mailnews/mime/src/mimetpfl.cpp b/mailnews/mime/src/mimetpfl.cpp
index 8d1dcbb527..d354217e55 100644
--- a/mailnews/mime/src/mimetpfl.cpp
+++ b/mailnews/mime/src/mimetpfl.cpp
@@ -277,7 +277,9 @@ static int MimeInlineTextPlainFlowed_parse_line(const char* aLine,
   const char* linep = real_line.BeginReading();
   // Space stuffed?
   if (' ' == *linep) {
+    line++;
     linep++;
+    length--;
   } else {
     // count '>':s before the first non-'>'
     while ('>' == *linep) {