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

Start with the supplied message-error.txt mailbox and the email.parser Parser snippet, then inspect multipart boundary handling in the standard library. Review linked PR gh-135891 for the work already underway. Done means the example is recognized as multipart and msg.is_multipart() returns True.