Exactly, I need to buffer the data from the E220 until the UART peripheral of the uC is ready. For the moment it is not possible to change to another module that uses a syncronous protocol. But you gave me a good idea for a temporary fix.
I'm avoiding using the RX module also as a transmitter due to power limitations, but I can send a first empty message that makes the E220 to wake up the uC, and after a couple seconds (when the uC is fully awake) send another message with the data. I don't have any power limitations on the TX side.