A variation of YGM an acronym for 'You've Got Mail' originating from AOL users due to the popularity of the default automated voice service audio sample played/heard upon receiving mail and later famed (maybe) by the film 'sleepless in Seatle' (I think).
YGMCS is a relatively new acronym standing for 'You've Got Mail Coming Soon' to let someone know you are about to email them soon (usually within 1hr).
1337 H4x0R - 'The job is done, where's my money?'.
n3wb13 - 'YGMCS with the payment & details for another job =)'.