Actually this is what is happening - currently using Mandrill.
Unfortunately “Hard bounces” from addresses already blocked by Mandrill appear to effect “Deliverability” and thus “Sends this hour” etc.
And obviously I setup email notification for email delivery faulires - I would like to avoid receiving these for addresses / users known to have a problem.
To be clear - not a big issue - just a little annoying - I’m attempting to reduce the noise from Discourse installations.