When it costs a significant fee to do something, you work very carefully to make sure that you've done everything right (aka "best practices"), so that you get a suitable return on investment. When something is close to free (aka e-mail marketing, on a variable cost basis), the discipline is different --- not bad, but different.

Challenge response systems have been around long enough now that pretty much everybody has an opinion on them. The end users who use challenge response systems seem to love them. But legitimate email senders often never respond to challenges, and so the end users are actually missing out on a lot of wanted email.

Don't fire nonresponders before asking first. Until you know why, you can't fix things.