Multipart with Content-Type-Encoding: 8bit

[kbouinyrs]

offlineimap fails to fetch one MIME-encoded message in my inbox. This message is a multipart/mixed containing two parts

• a multipart/alternative (plain text and HTML)
• a PDF attachment

Both alternatives of the first part use the following headers

Content-Type: text/plain; charset=UTF-8
Content-Transfer-Encoding: 8bit

and the content uses German umlauts, for example

die Anzahl der Nachfragen steigt und wir freuen uns, Sie darüber informieren zu können, dass wir den 
Rechnungsversand umstellen: Künftig erhalten Sie Ihre Rechnung ressourcenschonend als PDF per E–Mail.
Sofern Sie mit der Umstellung einverstanden sind und Ihre E–Mailadresse gültig ist, müssen Sie nichts weiter tun.

Thunderbird does not have issues with this message.

I attached the failing message. I slightly anonymized it, and replaced the PDF with a simple dummy text. When I removed the attachment completely (so that only the multipart/alternative remained), the message was fetched correctly.

Test.tar.gz

offlineimap v8.0.0, imaplib2 v3.06, Python v3.12.7, OpenSSL 3.4.0 22 Oct 2024

on Manjaro

Error message:

'ascii' codec can't encode characters in position 366-367: ordinal not in range(128)

Traceback:
File "/usr/lib/python3.12/site-packages/offlineimap/folder/Base.py", line 815, in copymessageto
    new_uid = dstfolder.savemessage(uid, message, flags, rtime)
            ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
File "/usr/lib/python3.12/site-packages/offlineimap/folder/Maildir.py", line 409, in savemessage
    tmpname = self.save_to_tmp_file(messagename, msg)
            ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
File "/usr/lib/python3.12/site-packages/offlineimap/folder/Maildir.py", line 359, in save_to_tmp_file
    fd.write(msg.as_bytes(policy=output_policy))
            ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
File "/usr/lib/python3.12/email/message.py", line 208, in as_bytes
    g.flatten(self, unixfrom=unixfrom)
File "/usr/lib/python3.12/email/generator.py", line 117, in flatten
    self._write(msg)
File "/usr/lib/python3.12/email/generator.py", line 182, in _write
    self._dispatch(msg)
File "/usr/lib/python3.12/email/generator.py", line 219, in _dispatch
    meth(msg)
File "/usr/lib/python3.12/email/generator.py", line 286, in _handle_multipart
    g.flatten(part, unixfrom=False, linesep=self._NL)
File "/usr/lib/python3.12/email/generator.py", line 117, in flatten
    self._write(msg)
File "/usr/lib/python3.12/email/generator.py", line 182, in _write
    self._dispatch(msg)
File "/usr/lib/python3.12/email/generator.py", line 219, in _dispatch
    meth(msg)
File "/usr/lib/python3.12/email/generator.py", line 278, in _handle_multipart
    self.write(subparts)
File "/usr/lib/python3.12/email/generator.py", line 420, in write
    self._fp.write(s.encode('ascii', 'surrogateescape'))
                ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^