Sorry, I think '<A@A.A.A A.A>' is not valid, as spaces are not allowed between words.

However I am not seeing the original unfolded source if there is an error, unless I am misunderstanding the API.

For example:

--- cut here ---
import email.header
import email.utils
import email.policy

def test(test):
    msg_string = f"Message-id: {test}"
    message = email.message_from_string(msg_string, policy=email.policy.default)
    out = message['Message-id']
    print(test)
    print(out)

test('<A@A.A.A A.A>') # invalid
test('<A@A.A.AA.A>') # valid
--- cut here ---

This produces:

<A@A.A.A A.A>
<A@A.A.A> # truncated at error
<A@A.A.AA.A>
<A@A.A.AA.A>

i.e. the invalid input is truncated