dll

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. Soft bounces can happen for all kinds of reasons - full servers, temporary processing issues, etc. I too have received them from time to time but it's usually after I've made a change to a booking.

lupine

If that was the cause I should have received a bunch of them -- before I switched email providers, Comcast regularly but intermittently turned away emails sent to me for no good reason. (Not even put in my spam folder -- simply refused.) It was especially likely when a large volume sender (like United) changed IP addresses, which Comcast viewed as a mighty suspicious action.

jcamp028

FlyingHighlander

I think this is plausible, at least for me. I received a paper confirmation last week after using UA miles to book an award flight on a partner. I never did get an email confirmation. A few days later the paper confirmation showed up.

mozilla

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.