I'm sure if the email was from a US-based office, far fewer people would call it a scam, albeit the chances of scam would be the same, if not higher because a higher total number of employed people means a higher probability of a rogue employee.

But it happens to be MX, and although UA has a big operation there, and that is where the OP is actually traveling to, which perfectly explains why the agent can access the reservation, this has to be a scam.

This is stereotyping which encourages prejudice, and it definitely calls for some introspection.