asEmailMessage() raises ValueError when message has multiple TO recipients

[glorat]

Bug Metadata

• Version of extract_msg: 0.55.0
• Your python version: Python 3.12
• How did you launch extract_msg?
  • My command line or
  • I used the extract_msg package

Describe the bug
Calling asEmailMessage() on a .msg file with multiple To recipients raises a ValueError from Python's email library. The root cause is that
msg.header (an email.message.Message) allows repeated keys, so a multi-recipient message produces two separate TO entries. The header-copying loop
assigns each one individually via ret[key] = value, but EmailMessage.__setitem__ enforces a maximum of one TO field.

What code did you use or can we use to reproduce this error?

import extract_msg

with extract_msg.openMsg("multi-to.msg") as msg:                                                                                                              
    em = msg.asEmailMessage()

Is there a message.msg file you want to share to help us reproduce this?

• Uploaded message (drag and drop on this window)

Traceback

File ".../extract_msg/msg_classes/message_base.py", line 171, in asEmailMessage
    ret[key] = value.replace('\r\n', '').replace('\n', '')
  File ".../email/message.py", line 439, in __setitem__                                                                                                       
    raise ValueError("There may be at most {} {} headers "
ValueError: There may be at most 1 TO headers in a message                                                                                                    

Additional context
The offending loop is in message_base.py around line 169:

for key, value in self.header.items():
    if key.lower() != 'content-type':
        ret[key] = value.replace('\r\n', '').replace('\n', '')

When there are multiple TO entries, the second assignment raises. The fix is to merge duplicate header values with a comma before assigning, which produces
valid RFC 5322 output (TO: alice@example.com, carol@example.com). A PR with the fix is forthcoming.

multi-to.msg