How do you parse mail envelope elements with aiosmtpd in Python?

I began with smtpd in order to process mailqueue, parse inbound emails and send them back to recipients (using smtpdlib.sendmail). I switched to aiosmtpd since i needed multithread processing (while smtpd is single-threaded, and besides that looks like discontinued).

By the way I'm puzzled by aiosmtpd management of mail envelope contents, that seems much more granular than before, so good if you need really fine tuning, but somewhat oversized if you just want to process body without modifying the rest.

To make an example, smtpd process_message method just needed data_decode=True parameter to process and decode mail body without touching anything, while aiosmtpd HANDLE_data method seems unable to automagically decode mail envelope and often gives exceptions with embedded images, attachments, and so on...

EDIT added code examples, smtpd first: following code will instantiate smtp server waiting for mail on port 10025 and delivering to 10027 via smtplib (both localhost). It is safe to work on data variable (basically perform string substitutions, my goal) for all kind of mail (text/html based, with embedded images, attachments...)

class PROXY_SMTP(smtpd.SMTPServer):
        def process_message(self, peer, mailfrom, rcpttos, data, decode_data=True):
        server = smtplib.SMTP('localhost', 10027)
        server.sendmail(mailfrom, rcpttos, data)
        server.quit()
server = PROXY_SMTP(('127.0.0.1', 10025), None)
asyncore.loop()

Previous code works well but in a single thread fashion (= 1 mail at once), so i switched to aiosmtpd to have concurrent mail processing. Same example with aiosmtpd would be roughly:

class MyHandler:
        async def handle_DATA(self, server, session, envelope):
                peer = session.peer
                mailfrom = envelope.mail_from
                rcpttos = envelope.rcpt_tos
                data = envelope.content.decode()
                server = smtplib.SMTP('localhost', 10027)
                server.sendmail(mailfrom, rcpttos, data)
                server.quit()

my_handler = MyHandler()

async def main(loop):
        my_controller = Controller(my_handler, hostname='127.0.0.1', port=10025)
        my_controller.start()
loop = asyncio.get_event_loop()
loop.create_task(main(loop=loop))
try:
     loop.run_forever()

This code works well for text emails, but will give exceptions when decoding envelope.content with any complex mail (mime content, attachments...)

How could I parse and decode mail text in aiosmtpd, perform string substitution as I did with smtpd, and reinject via smtplib? Thanks

--- SOLVED ---

This is what i gotten so far, minor adjustments are still needed (mainly for mime content separate handling and "rebuilding") but this solves my main problem: receive mail on separated threads, make room for text processing, sleep for fixed amount of time before final delivery. Thanks to tripleee answers and comments I found correct way.

    import asyncio
    from aiosmtpd.controller import Controller
    import smtplib
    from email import message_from_bytes
    from email.policy import default
            class MyHandler:
                async def handle_DATA(self, server, session, envelope):
                    peer = session.peer
                    mailfrom = envelope.mail_from
                    rcpttos = envelope.rcpt_tos
                    message = message_from_bytes(envelope.content, policy=default)
                    #HERE MAYBE WOULD BE SAFER TO WALK CONTENTS AND PARSE/MODIFY ONLY MAIL BODY, BUT NO SIDE EFFECTS UNTIL NOW WITH MIME, ATTACHMENTS...
                    messagetostring = message.as_string() ### smtplib.sendmail WANTED BYTES or STRING, NOT email OBJECT.
                    ### HERE HAPPENS TEXT PROCESSING, STRING SUBSTITUTIONS...
                    ### THIS WAS MY CORE NEED, ASYNCWAIT ON EACH THREAD
                    await asyncio.sleep(15)
                    server = smtplib.SMTP('localhost', 10027)
                    server.send_message(mailfrom, rcpttos, messagetostring) ### NEEDED TO INVERT ARGS ORDER
                    server.quit()
                    return '250 OK' ### ADDED RETURN

             my_handler = MyHandler()

             async def main(loop):
                    my_controller = Controller(my_handler, hostname='127.0.0.1', port=10025)
                    my_controller.start()
             loop = asyncio.get_event_loop()
             loop.create_task(main(loop=loop))
             try:
                    loop.run_forever()