Simple Python 3 script to convert eml Email files to mbox files.

eml2mbox.py

"""
Simple Python 3 script to convert eml Email files to mbox files.
Convert script requires only The Python Standard Libraries.

This script is useful to convert eml files exported from
E-Mail Accounts to mbox files. Mbox files can be imported
in common email clients such as Outlook, Thunderbird, or
Gnome Evolution.

Placing eml files to be converted to mbox files:
 * place all your eml files into the "in" directory:
   e.g. in/username_at_email_domain/inbox/123456.eml

Output (mbox-out):
 * mbox file for each of the discovered subdirectories with eml files.
"""


import re
import os
import mailbox
from email import policy
from email.parser import BytesParser


eml_regex = re.compile('.*\\.eml')

for directory in ['in', 'mbox-out']:
    if not os.path.exists(directory):
        os.makedirs(directory)

for root, dirs, files in os.walk("in", topdown=True):
    eml_files = list(filter(eml_regex.match, files))
    if eml_files:
        output_mbox_file = f'mbox-out/{root.strip("./").replace("/", "-")}.mbox'
        if os.path.exists(output_mbox_file):
            print(f"WARN: Deleting existing mbox: {output_mbox_file}")
            os.remove(output_mbox_file)

        print(f'Creating {output_mbox_file} from {len(eml_files)} eml files ...\n')

        destination = mailbox.mbox(output_mbox_file)
        destination.lock()

        for eml_file in eml_files:
            eml_file_path = os.path.join(root, eml_file)
            with open(eml_file_path, 'rb') as f:
                msg = BytesParser(policy=policy.default).parse(f)
                destination.add(mailbox.MHMessage(msg))

        destination.flush()
        destination.unlock()