Banks in Mexico including Azteca are no longer allowed by the Federal Bank of Mexico to accept ANY repeat ANY American denomination Bills for exchange to Mexican Pesos. Only the privately held "Casa de Cambio's" are permitted to make any such transactions as of about one week ago. Just FYI.
The CitiBanamex branch that I have my account accepts Dollars, Euros, and all other currency and I can purchase Euros with Dollars or Pesos, you do need to register with the bank in order to exchange dollars at a bank, that is a measure established for controlling the laundering of narco profits, the exchange houses do have a limit of the amount you can excange.
For those of you that do not have a bank account in Mexico use the ATM machines at a bank, stay away from the machines in stores or shopping centers and try to have a second person accompany you to block what you do so that the bad elements do not see what you are doing, read the warning on counterfeit 500.00 pesos.