Mystery Man

 is an email in Prey. The email was added as the result of a "get your name in the game" contest, which was won by Gus Lanzetta (https://twitter.com/GusLanzetta).

See https://bethesda.net/en/article/65eA0nCPQWUkmi8ugkeaEw/prey-i-am-transtar-npc-contest

Location
This email can be read on Rory Manion's workstation in Behavioral Biometrics, Psychotronics.