My guess - from years of experience managing email programs - is that when their reservation system detects even a soft bounce on an email address it automatically sends out the printed itinerary.
With so many emails getting 451'd, I'd expect that UA would have long abandoned this practice if that was the case. I'm not saying your suggestion is impossible, but if it was the case, then it's definitely more than just a soft bounce. Maybe it's one or more specific SMTP reply codes that are seldom used that trigger the letter.
IMHO I still believe this is some kind of light fraud countermeasure, triggered when something is off but nothing that is in itself too suspicious (login from a weird IP, recent email address change, unusual travel pattern, CC AVS mismatch, ...). The reason I believe that is because UA would only make an extra expense to protect itself from something, not to protect you from not getting your e-ticket confirmation.