No email, but entering my number turned up:
Milestone 1: spend $2,500 and earn 10,000 bonus award miles
Milestone 2: spend $3,000 total and earn 15,000 more award miles
Milestone 3: spend $4,500 total and earn 25,000 more award miles
This probably will affect my buying, as I have mostly been flying partners and crediting to MP (I'm at 37K PQM on 450 PQD) and I think I can hit the third tier by November. Better MP earnings has been a significant factor in my giving money to LH, NH, BR instead of UA.