I see no reason why this wouldn't be legit if:

1. The OP held the reservation to be later ticketed at UA ticket office
2. The reservation had travel to/from MEX, likely causing the PNR to show up at the local office as 'not ticketed'
3. The OP emailed [email protected] back (not a spoofed reply-to) and received a response.
4. It's totally possible the fare dropped during the time of holding the ticket and contact being made. Maybe the office wanted to help out, I don't see why that is so far-fetched.

I would have no problems contacting the office after a quick search of the phone number in this case. Plus, my cc will cover me for fraud if anything were to happen.