Thumbs up to this request.
When we hit the Rate-Limits on some RBLs (especially near peak mail loads such as Valentine's, Mother's Day, and Black Friday) the mail spool in SmarterMail would seemingly just stop responding and restarting the service wouldn't get mail flowing no matter how many times we tried. It was only when we glanced at the Antispam Settings that we would discover that an RBL was timing out or had an excessively long response time alerting us that we should disable it to get the mail spool running again. It is really, really useful and should be readded, if possible, to v16.