If you also have a Centurion or Platinum card you will receive a postal letter from American Express
I (also a Plat Card holder) actually received a different letter (of a form I haven't seen before when applying for new MR earning cards):
"We are writing to confirm your recent request for changes to your American Express card account. As a result of your request to make a change to your account, there are additional fees that will be charged for your enrollment in the program
The following charges will appear on your next billing statement:
Membership Rewards program Annual Enrollment fee $0.00
Total net fee change: 0.00"