This is like saying because their services suck, then I should downgrade my workflow accordingly.. even though I have never came across a service that will deny

This is like saying because their services suck, then I should downgrade my workflow accordingly.. even though I have never came across a service that will deny unknown domain, but if they do, I will just walk away, it gives you an idea what service is that anyway. For the QA part, deal with it as if it’s a company email, definitely it won’t be at gmail or yahoo.