This is a SCAM period (and if you believe the email then I have a bridge in Brooklyn for sale - ). I have yet to hear of an international corporation sending me an email and asking me to call them in Mexico with a credit card number so that I can save $50/ticket. If it really was legitimately United then they would have asked you to call their Customer Service number where are all ticketing issues are handled and not some number in Mexico. On a related issue, if you have many United miles or other United credits I would monitor them carefully as they obviously have access to your information and could quickly empty your Mileage Plus account.

I just did a quick lookup and the minimum wage in Mexico is $4.71/day. Thousands and thousands of such emails can be sent out and even if the response is less than 1% the scammers can make $$$ however it is more likely that it is the CC number that they are after.