That email is triggered when you book a ticket with no US DM number. It's genuine.
It's possible someone accidentally entered the wrong email address in the email box, and it got to you.
e.g.:
There are miles of opportunities. Don't miss out!
Confirmation code: XXXXXX
Departure date: 01-XX-2010
Thanks for buying a ticket with US.
We owe each person on your itinerary 2,706 miles – but each person needs to have a Dividend Miles account to get them!
Don't let your miles go to waste:
1. If you don't have an account, join Dividend Miles now. It's free and will only take a few minutes.
2. If you do, look up your reservation with confirmation code XXXXXX add your Dividend Miles number.
Remember, you can only claim miles for yourself and the name on your ticket must be the same name on your Dividend Miles account. Once you're all set up, we'll post your miles within 5 days of your travel.
Get up to 35,000 more miles with a US Airways MasterCard.®
Plus, you can take up to two guests with you on your next trip for only $99 each (plus taxes and fees).
Learn more or apply online
HTSC