“If I had asked people what they wanted, they would...

> My suspicion with the TSB incident, is either insufficient testing

I don't think such a thing as "sufficient testing" exists in IT anymore.

When was the last time you saw a...

Here it is - DevOps Car:

And having a calendar table in your possession, you don't need to count Sat-Sun switches, you simply count the days marked as business days in the calendar.

But actually - what do you expect from the industry driven by continos delivery?

Waterfall goes down regardless of the testing results.
When did you last time see a...

Why do you need a table parameter?

Can you describe your scenario which validates it's use?

Do you need "business days" or "week days"?

There are public holidays which are not business days, and might not fall on weekends.

There is a fundamental error in the design described in the op.

Every customer with an email address is considered a recipient of emails.

Except those ones which are excluded.

Having an email...

No, I'd usually compare to what should not be sent WHILE generating the list to be sent.

And those which not to be sent never appear in the list to be...