it actually didn’t exist to cause me to specify one or more of these settings.
my script was always respecting the Retry-After header, as we can see from original line 101;
retry_after = r.headers.get("Retry-After")
however this header was becoming unnecessarily large as i schedule different service units for this same script to run at different times on a day.
Therefore, multiples of some of these settings from the default; improved email deliverability via mail-receiver which remains important.