I'm really curious about this. The question (highlighed in bold):
Can someone name ONE GOOD REASON to have their email server permanently set up to auto-fail/immediate-fail on 4xx codes?
Because frankly, it sounds like "their" setups are broken out-of-the-box. SMTP is not Instant Messaging. Stop treating it like IRC or Jabber or MSN or insert-IM-technology-here. I don't know what possesses people to have the "IMMEDIATE DELIVERY OR FAIL" mentality with SMTP setups, but they need to stop doing that. It just plain breaks things. Every two or three years, I stumble into this. Someone, somewhere, has decided in their infinite wisdom that 4xx codes are immediate failures, and suddenly its OMGWTFBBQ THE INTARNETZ ARE BORKEN, HALP SKY IS FALLING instead of "oh, it'll re-attempt delivery in about 30 minutes". It amazes me how it suddenly becomes "my" problem that a message won't go through, because someone else misconfigured "their" SMTP service.
IF there is a legitimate reason for having your server permanently set up in this manner, then the first good answer will get the check.
IF there is no good reason (and I suspect there isn't), then the first good-sounding-if-still-logically-flawed answer will get the check.