[Postfix] Change mail_name to Postcow and only replace headers when mail_name matches

data/conf/postfix/mailcow_anonymize_headers.pcre

@@ -1,5 +1,8 @@
+if /^\s*Received:.*Authenticated sender.*\(Postcow\)/
 /^\s*Received:.*Authenticated sender:(.+)/
-    REPLACE Received: from localhost (localhost [127.0.0.1]) (Authenticated sender:$1
+  REPLACE Received: from localhost (localhost [127.0.0.1]) (Authenticated sender:$1
+endif
+
 /^\s*User-Agent/        IGNORE
 /^\s*X-Enigmail/        IGNORE
 /^\s*X-Mailer/          IGNORE