from email import message_from_bytes
#from email.parser import BytesParser
from email.policy import default
##from cProfile import run as cprun

b = (b'''From: me <self@example.com>
To: you <recipient@example.net>
Subject: sample with ;;;;;;;;; stuffing in MIME Content-Type: header
Content-type: text/plain''' + b';' * 54 + b'\n' +
               36 * (b' ' + b';' * 990 + b'\n') + b'''\
Content-transfer-encoding: 7bit
MIME-Version: 1.0

Hello.
''')
## cprun('message_from_bytes(b, policy=default)', sort=1)

m = message_from_bytes(b, policy=default)
#m = BytesParser(policy=default).parsebytes(b)

print(m.as_string())