diff --git a/git-send-email.perl b/git-send-email.perl
index 5bf4307442b..cb6dca2500c 100755
--- a/git-send-email.perl
+++ b/git-send-email.perl
@@ -1788,7 +1788,8 @@ EOF
 		if (is_outlook($smtp_server)) {
 			if ($smtp->message =~ /<([^>]+)>/) {
 				$message_id = "<$1>";
-				printf __("Outlook reassigned Message-ID to: %s\n"), $message_id;
+				$header =~ s/^(Message-ID:\s*).*\n/${1}$message_id\n/m;
+				printf __("Outlook reassigned Message-ID to: %s\n"), $message_id if $smtp->debug;
 			} else {
 				warn __("Warning: Could not retrieve Message-ID from server response.\n");
 			}