diff --git a/src/mb.kludge.pas b/src/mb.kludge.pas
index 818e81f..83d12ac 100644
--- a/src/mb.kludge.pas
+++ b/src/mb.kludge.pas
@@ -282,7 +282,11 @@ begin
   AppendKludge(Result, 'REPLY:', attrs.Get('replyid', ''), True);
   AppendKludge(Result, 'PID:',   attrs.Get('pid',     ''), True);
   AppendKludge(Result, 'TID:',   attrs.Get('tid',     ''), True);
-  AppendKludge(Result, 'FLAGS:', attrs.Get('flags',   ''), True);
+  { FTS-4001: FLAGS kludge uses a space separator, NOT a colon.
+    (fmail's jamfun.c:321 and ftr.c:113 both emit "\1FLAGS <flags>\r"
+    with no colon.  FTS-4001 §4 Kludge Line Form matches.)  Bare
+    name + single space so AppendKludge's ' ' join lands correctly. }
+  AppendKludge(Result, 'FLAGS',  attrs.Get('flags',   ''), True);
   AppendKludge(Result, 'CHRS:',  attrs.Get('chrs',    ''), True);
   AppendKludge(Result, 'TZUTC:', attrs.Get('tzutc',   ''), True);
   AppendKludgeLines(Result, 'Via', attrs.Get('via',   ''), True);