diff --git a/mailnews/imap/public/nsIImapFlagAndUidState.idl b/mailnews/imap/public/nsIImapFlagAndUidState.idl
index e78c26080b..bf74e4d433 100644
--- a/mailnews/imap/public/nsIImapFlagAndUidState.idl
+++ b/mailnews/imap/public/nsIImapFlagAndUidState.idl
@@ -70,5 +70,7 @@ interface nsIImapFlagAndUidState : nsISupports
    */
   ACString getCustomAttribute(in unsigned long aUid,
                               in ACString aCustomAttributeName);
+  void setOtherKeywords(in unsigned short index, in AUTF8String otherKeyword);
+  AUTF8String getOtherKeywords(in unsigned short index);
 };
 
diff --git a/mailnews/imap/src/nsImapFlagAndUidState.cpp b/mailnews/imap/src/nsImapFlagAndUidState.cpp
index 7aef4e8d7b..650cef3e73 100644
--- a/mailnews/imap/src/nsImapFlagAndUidState.cpp
+++ b/mailnews/imap/src/nsImapFlagAndUidState.cpp
@@ -110,6 +110,27 @@ nsImapFlagAndUidState::GetSupportedUserFlags(uint16_t *aFlags)
   return NS_OK;
 }
 
+NS_IMETHODIMP
+nsImapFlagAndUidState::SetOtherKeywords(uint16_t index, const nsACString &otherKeyword)
+{
+  if (index == 0)
+    fOtherKeywords.Clear();
+  nsAutoCString flag(otherKeyword);
+  ToLowerCase(flag);
+  fOtherKeywords.AppendElement(flag);
+  return NS_OK;
+}
+
+NS_IMETHODIMP
+nsImapFlagAndUidState::GetOtherKeywords(uint16_t index, nsACString &aKeyword)
+{
+  if (index < fOtherKeywords.Length())
+    aKeyword = fOtherKeywords[index];
+  else
+    aKeyword = EmptyCString();
+  return NS_OK;
+}
+
 // we need to reset our flags, (re-read all) but chances are the memory allocation needed will be
 // very close to what we were already using
 
diff --git a/mailnews/imap/src/nsImapFlagAndUidState.h b/mailnews/imap/src/nsImapFlagAndUidState.h
index 61a64902c9..3b7f1c1188 100644
--- a/mailnews/imap/src/nsImapFlagAndUidState.h
+++ b/mailnews/imap/src/nsImapFlagAndUidState.h
@@ -51,6 +51,8 @@ private:
     bool                    fPartialUIDFetch;
     uint32_t                fNumAdded;
     bool                    fStartCapture;
+    // Keywords (aka, tags) in FLAGS response to SELECT defined by other clients
+    nsTArray<nsCString>     fOtherKeywords;
     mozilla::Mutex mLock;
 };
 
diff --git a/mailnews/imap/src/nsImapMailFolder.cpp b/mailnews/imap/src/nsImapMailFolder.cpp
index 0b204b8e94..1b697435c6 100644
--- a/mailnews/imap/src/nsImapMailFolder.cpp
+++ b/mailnews/imap/src/nsImapMailFolder.cpp
@@ -100,6 +100,7 @@ static NS_DEFINE_CID(kCImapHostSessionList, NS_IIMAPHOSTSESSIONLIST_CID);
 extern mozilla::LazyLogModule gAutoSyncLog; // defined in nsAutoSyncManager.cpp
 extern mozilla::LazyLogModule IMAP;         // defined in nsImapProtocol.cpp
 extern mozilla::LazyLogModule IMAP_CS;      // For CONDSTORE, defined in nsImapProtocol.cpp
+mozilla::LazyLogModule IMAP_KW("IMAP_KW");  // for logging keyword (tag) processing
 
 #define MAILNEWS_CUSTOM_HEADERS "mailnews.customHeaders"
 
@@ -4440,7 +4441,7 @@ void nsImapMailFolder::TweakHeaderFlags(nsIImapProtocol* aProtocol, nsIMsgDBHdr
       if (newFlags)
         tweakMe->OrFlags(newFlags, &dbHdrFlags);
       if (!customFlags.IsEmpty())
-        (void) HandleCustomFlags(m_curMsgUid, tweakMe, userFlags, customFlags);
+        (void) HandleCustomFlags(m_curMsgUid, tweakMe, userFlags, customFlags, nullptr);
     }
   }
 }
@@ -4751,7 +4752,8 @@ nsImapMailFolder::BeginMessageUpload()
 nsresult nsImapMailFolder::HandleCustomFlags(nsMsgKey uidOfMessage,
                                              nsIMsgDBHdr *dbHdr,
                                              uint16_t userFlags,
-                                             nsCString &keywords)
+                                             nsCString &keywords,
+                                             nsIImapFlagAndUidState *flagState)
 {
   nsresult rv = GetDatabase();
   NS_ENSURE_SUCCESS(rv, rv);
@@ -4785,6 +4787,71 @@ nsresult nsImapMailFolder::HandleCustomFlags(nsMsgKey uidOfMessage,
     if (existingProperty.IsEmpty())
       dbHdr->SetStringProperty("junkscoreorigin", "imapflag");
   }
+
+  if (flagState && !(userFlags & kImapMsgSupportUserFlag))
+  {
+    nsCString localKeywords;
+    if (!(userFlags & kImapMsgSupportUserFlag))
+    {
+      dbHdr->GetStringProperty("keywords", getter_Copies(localKeywords));
+      MOZ_LOG(IMAP_KW, mozilla::LogLevel::Debug, ("UID=%" PRIu32 ", localKeywords=|%s| rcvdKeyword=|%s|",
+              uidOfMessage, localKeywords.get(), keywords.get()));
+    }
+    nsTArray<nsCString> localKeywordArray;
+    nsTArray<nsCString> rcvdKeywordArray;
+    ParseString(localKeywords, ' ', localKeywordArray);
+    ParseString(keywords, ' ', rcvdKeywordArray);
+
+    nsAutoCString mozLogDefinedKWs;
+    if (MOZ_LOG_TEST(IMAP_KW, mozilla::LogLevel::Debug))
+      mozLogDefinedKWs.AppendLiteral("Defined keywords = |");
+    uint32_t i = 0;
+    while (true)
+    {
+      nsAutoCString definedKeyword;
+      flagState->GetOtherKeywords(i++, definedKeyword);
+      if (definedKeyword.IsEmpty())
+      {
+        if (MOZ_LOG_TEST(IMAP_KW, mozilla::LogLevel::Debug))
+        {
+          mozLogDefinedKWs.Append('|');
+          MOZ_LOG(IMAP_KW, mozilla::LogLevel::Debug, ("%s", mozLogDefinedKWs.get()));
+        }
+        break;
+      }
+
+      if (MOZ_LOG_TEST(IMAP_KW, mozilla::LogLevel::Debug))
+      {
+        mozLogDefinedKWs.Append(definedKeyword.get());
+        mozLogDefinedKWs.Append(' ');
+      }
+
+      bool inLocal = localKeywordArray.Contains(definedKeyword);
+      bool inRcvd = rcvdKeywordArray.Contains(definedKeyword);
+      if (inLocal && inRcvd)
+        rcvdKeywordArray.RemoveElement(definedKeyword);
+      if (inLocal && !inRcvd)
+        localKeywordArray.RemoveElement(definedKeyword);
+    }
+    // Combine local and rcvd keyword arrays into a single string
+    // so it can be passed to SetStringProperty(). If element of
+    // local already in rcvd, avoid duplicates in combined string.
+    nsAutoCString combinedKeywords;
+    for (i = 0; i < localKeywordArray.Length(); i++)
+    {
+      if (!rcvdKeywordArray.Contains(localKeywordArray[i]))
+      {
+        combinedKeywords.Append(localKeywordArray[i]);
+        combinedKeywords.Append(' ');
+      }
+    }
+    for (i = 0; i < rcvdKeywordArray.Length(); i++)
+    {
+      combinedKeywords.Append(rcvdKeywordArray[i]);
+      combinedKeywords.Append(' ');
+    }
+    return dbHdr->SetStringProperty("keywords", combinedKeywords.get());
+  }
   return (userFlags & kImapMsgSupportUserFlag) ?
           dbHdr->SetStringProperty("keywords", keywords.get()) : NS_OK;
 }
@@ -4830,7 +4897,7 @@ nsresult nsImapMailFolder::SyncFlags(nsIImapFlagAndUidState *flagState)
 
     nsCString keywords;
     if (NS_SUCCEEDED(flagState->GetCustomFlags(uidOfMessage, getter_Copies(keywords))))
-        HandleCustomFlags(uidOfMessage, dbHdr, supportedUserFlags, keywords);
+        HandleCustomFlags(uidOfMessage, dbHdr, supportedUserFlags, keywords, flagState);
 
     NotifyMessageFlagsFromHdr(dbHdr, uidOfMessage, flags);
   }
@@ -4935,7 +5002,7 @@ nsImapMailFolder::NotifyMessageFlags(uint32_t aFlags,
       GetSupportedUserFlags(&supportedUserFlags);
       NotifyMessageFlagsFromHdr(dbHdr, aMsgKey, aFlags);
       nsCString keywords(aKeywords);
-      HandleCustomFlags(aMsgKey, dbHdr, supportedUserFlags, keywords);
+      HandleCustomFlags(aMsgKey, dbHdr, supportedUserFlags, keywords, nullptr);
     }
   }
   return NS_OK;
diff --git a/mailnews/imap/src/nsImapMailFolder.h b/mailnews/imap/src/nsImapMailFolder.h
index 3c9bf7c516..45a47a8fc0 100644
--- a/mailnews/imap/src/nsImapMailFolder.h
+++ b/mailnews/imap/src/nsImapMailFolder.h
@@ -354,7 +354,8 @@ protected:
 
   nsresult SyncFlags(nsIImapFlagAndUidState *flagState);
   nsresult HandleCustomFlags(nsMsgKey uidOfMessage, nsIMsgDBHdr *dbHdr,
-                             uint16_t userFlags, nsCString& keywords);
+                             uint16_t userFlags, nsCString& keywords,
+                             nsIImapFlagAndUidState *flagState);
   nsresult NotifyMessageFlagsFromHdr(nsIMsgDBHdr *dbHdr, nsMsgKey msgKey,
                                      uint32_t flags);
 
diff --git a/mailnews/imap/src/nsImapServerResponseParser.cpp b/mailnews/imap/src/nsImapServerResponseParser.cpp
index 91b78714e3..9243066ff0 100644
--- a/mailnews/imap/src/nsImapServerResponseParser.cpp
+++ b/mailnews/imap/src/nsImapServerResponseParser.cpp
@@ -782,7 +782,7 @@ void nsImapServerResponseParser::mailbox_data()
     if (fGotPermanentFlags)
       skip_to_CRLF();
     else
-      parse_folder_flags();
+      parse_folder_flags(true);
   }
   else if (!PL_strcasecmp(fNextToken, "LIST") ||
            !PL_strcasecmp(fNextToken, "XLIST"))
@@ -1850,20 +1850,19 @@ void nsImapServerResponseParser::text()
   skip_to_CRLF();
 }
 
-void nsImapServerResponseParser::parse_folder_flags()
+void nsImapServerResponseParser::parse_folder_flags(bool calledForFlags)
 {
   uint16_t labelFlags = 0;
+  bool storeUserFlags = !(fSupportsUserDefinedFlags & kImapMsgSupportUserFlag) &&
+                        calledForFlags && fFlagState;
+  uint16_t numOtherKeywords = 0;
 
   do
   {
     AdvanceToNextToken();
     if (*fNextToken == '(')
       fNextToken++;
-    if (!PL_strncasecmp(fNextToken, "$MDNSent", 8))
-      fSupportsUserDefinedFlags |= kImapMsgSupportMDNSentFlag;
-    else if (!PL_strncasecmp(fNextToken, "$Forwarded", 10))
-      fSupportsUserDefinedFlags |= kImapMsgSupportForwardedFlag;
-    else if (!PL_strncasecmp(fNextToken, "\\Seen", 5))
+    if (!PL_strncasecmp(fNextToken, "\\Seen", 5))
       fSettablePermanentFlags |= kImapMsgSeenFlag;
     else if (!PL_strncasecmp(fNextToken, "\\Answered", 9))
       fSettablePermanentFlags |= kImapMsgAnsweredFlag;
@@ -1873,23 +1872,52 @@ void nsImapServerResponseParser::parse_folder_flags()
       fSettablePermanentFlags |= kImapMsgDeletedFlag;
     else if (!PL_strncasecmp(fNextToken, "\\Draft", 6))
       fSettablePermanentFlags |= kImapMsgDraftFlag;
-    else if (!PL_strncasecmp(fNextToken, "$Label1", 7))
-      labelFlags |= 1;
-    else if (!PL_strncasecmp(fNextToken, "$Label2", 7))
-      labelFlags |= 2;
-    else if (!PL_strncasecmp(fNextToken, "$Label3", 7))
-      labelFlags |= 4;
-    else if (!PL_strncasecmp(fNextToken, "$Label4", 7))
-      labelFlags |= 8;
-    else if (!PL_strncasecmp(fNextToken, "$Label5", 7))
-      labelFlags |= 16;
     else if (!PL_strncasecmp(fNextToken, "\\*", 2))
     {
+      // User defined and special keywords (tags) can be defined and set for
+      // mailbox. Should only occur in PERMANENTFLAGS response.
       fSupportsUserDefinedFlags |= kImapMsgSupportUserFlag;
       fSupportsUserDefinedFlags |= kImapMsgSupportForwardedFlag;
       fSupportsUserDefinedFlags |= kImapMsgSupportMDNSentFlag;
       fSupportsUserDefinedFlags |= kImapMsgLabelFlags;
     }
+    else
+    {
+      // Treat special and built-in $LabelX's as user defined if a
+      // save occurs below.
+      if (!PL_strncasecmp(fNextToken, "$MDNSent", 8))
+        fSupportsUserDefinedFlags |= kImapMsgSupportMDNSentFlag;
+      else if (!PL_strncasecmp(fNextToken, "$Forwarded", 10))
+        fSupportsUserDefinedFlags |= kImapMsgSupportForwardedFlag;
+      else if (!PL_strncasecmp(fNextToken, "$Label1", 7))
+        labelFlags |= 1;
+      else if (!PL_strncasecmp(fNextToken, "$Label2", 7))
+        labelFlags |= 2;
+      else if (!PL_strncasecmp(fNextToken, "$Label3", 7))
+        labelFlags |= 4;
+      else if (!PL_strncasecmp(fNextToken, "$Label4", 7))
+        labelFlags |= 8;
+      else if (!PL_strncasecmp(fNextToken, "$Label5", 7))
+        labelFlags |= 16;
+
+      // Save user keywords defined for mailbox, usually by other clients.
+      // But only do this for FLAGS response, not PERMANENTFLAGS response
+      // and if '\*' has not appeared in a PERMANENTFLAGS response.
+      if (storeUserFlags && *fNextToken != '\r')
+      {
+        if (*(fNextToken + strlen(fNextToken) - 1) != ')')
+        {
+          // Token doesn't end in ')' so save it as is.
+          fFlagState->SetOtherKeywords(numOtherKeywords++, nsDependentCString(fNextToken));
+        }
+        else
+        {
+          // Token ends in ')' so end of list. Change ')' to null and save.
+          fFlagState->SetOtherKeywords(numOtherKeywords++,
+                                       nsDependentCSubstring(fNextToken, strlen(fNextToken) - 1));
+        }
+      }
+    }
   } while (!fAtEndOfLine && ContinueParse());
 
   if (labelFlags == 31)
@@ -1947,7 +1975,7 @@ void nsImapServerResponseParser::resp_text_code()
       uint32_t saveSettableFlags = fSettablePermanentFlags;
       fSupportsUserDefinedFlags = 0;  // assume no unless told
       fSettablePermanentFlags = 0;            // assume none, unless told otherwise.
-      parse_folder_flags();
+      parse_folder_flags(false);
       // if the server tells us there are no permanent flags, we're
       // just going to pretend that the FLAGS response flags, if any, are
       // permanent in case the server is broken. This will allow us
diff --git a/mailnews/imap/src/nsImapServerResponseParser.h b/mailnews/imap/src/nsImapServerResponseParser.h
index af8dc48ff2..9072114d97 100644
--- a/mailnews/imap/src/nsImapServerResponseParser.h
+++ b/mailnews/imap/src/nsImapServerResponseParser.h
@@ -137,7 +137,7 @@ protected:
   virtual void    resp_cond_state(bool isTagged);
   virtual void    text_mime2();
   virtual void    text();
-  virtual void    parse_folder_flags();
+  virtual void    parse_folder_flags(bool calledForFlags);
   virtual void    enable_data();
   virtual void    language_data();
   virtual void    authChallengeResponse_data();