standard library email.parser fails to recognize multipart message

Bug report

Bug description:

If an email message with attachments contain a boundary with the characters "<--- ... --->" the email.parser module fails to recognize the message as being multipart.

I have attached to this bug report an example mailbox file, and when the following snippet is run on it you will see that is_multipart() returns False, although the message is clearly a multipart message.

I found this boundary string used in a real life email I received.

from email.parser import Parser

# Create parser instance
P = Parser()

with open("message-error.mbox") as f:
    msg = P.parse(f)

print(msg.is_multipart())

message-error.txt

CPython versions tested on:

3.13

Operating systems tested on:

Linux

Linked PRs

• gh-135891

Inizia con la mailbox message-error.txt fornita e lo snippet email.parser Parser, quindi esamina la gestione dei boundary multipart nella libreria standard. Esamina la PR collegata gh-135891 per il lavoro già in corso. Il lavoro è completato quando l’esempio viene riconosciuto come multipart e msg.is_multipart() restituisce True.