The SMTP preprocessor is an SMTP decoder for user applications. Given a data buffer, SMTP will decode the buffer and find SMTP commands and responses. It will also mark the command, data header data body sections, and TLS data.
SMTP handles stateless and stateful processing. It saves state between individual packets. However maintaining correct state is dependent on the reassembly of the client side of the stream (ie, a loss of coherent stream data results in a loss of state).