async_bounce_rendering — which thread pool to use for bounce rendering
Async_Bounce_Rendering = true
When a message fails to be delivered due to a timeout, a 5XX error from the remote MTA or a
ec_reject in a Sieve script, Momentum generates and injects a bounce message. If async_bounce_rendering is true, this job is done in the IO thread pool, otherwise it gets done in the scheduler before returning control to the code that noted the bounce, namely the outbound SMTP, the Sieve script or the mail queue maintainer.
The default value for this option is
Async_Bounce_Rendering is valid in the global scope.