diff --git a/remail/mail.py b/remail/mail.py
index 440eecc..4ec3eaa 100644
--- a/remail/mail.py
+++ b/remail/mail.py
@@ -195,12 +195,10 @@ def msg_set_header(msg, hdr, txt):
     # come with wreckaged headers.
     txt = re_rmlfcr.sub(' ', txt)
 
-    for k in msg.keys():
-        if hdr.lower() == k.lower():
-            msg.replace_header(k, txt)
-            return
-    # Not found set new
-    msg[hdr] = txt
+    if msg.get(hdr):
+        msg.replace_header(hdr, txt)
+    else:
+        msg[hdr] = txt
 
 payload_valid_mime_headers = [
     'Content-Description',