Combine a directory of .eml files into a single Unix "mbox" file.

emlToMbox.py

#!/usr/bin/env python3

""" Converts a directory full of .eml files to a single Unix "mbox" file.
This is similar to http://www.cosmicsoft.net/emlxconvert.html

Accepts as input either an individual .eml file or a directory containing one
or more .eml files.

Usage:
$ ./emlToMbox.py inputdir/ output.mbox
$ ./emlToMbox.py input.eml output.mbox

STATUS:  Lightly tested using Python 3.9.1 
"""

import os
import sys
import mailbox

global debug
debug = True

def main( arguments ):
    infile_name = arguments[1]
    dest_name = arguments[2]
    
    if debug:
        print("Input is:  " + infile_name)
        print("Output is: " + dest_name)
    
    dest_mbox = mailbox.mbox(dest_name, create=True) # if dest doesn't exist create it
    dest_mbox.lock() # lock the mbox file
    
    if os.path.isdir(infile_name):
        if debug:
            print("Detected directory as input, using directory mode")
        count = 0
        for filename in os.listdir(infile_name):
            if filename.split('.')[-1] == "eml":
                try:
                    fi = open(os.path.join(infile_name, filename), 'r')
                except:
                    sys.stderr.write("Error while opening " + filename + "\n")
                    dest_mbox.close()
                    raise
                addFileToMbox( fi, dest_mbox )
                count += 1
                fi.close()
        if debug:
            print("Processed " + str(count) + " total files.")
    
    if infile_name.split('.')[-1] == "eml":
        if debug:
            print("Detected .eml file as input, using single file mode")
        try:
            fi = open(infile_name, 'r')
        except:
            sys.stderr.write("Error while opening " + infile_name + "\n")
            dest_mbox.close()
            raise
        addFileToMbox( fi, dest_mbox )
        fi.close()
    
    dest_mbox.close() # close/unlock the mbox file
    return 0

def addFileToMbox( fi, dest_mbox ):
    # Any additional preprocessing logic goes here...
    try:
        dest_mbox.add( fi )
    except:
        dest_mbox.close()
        raise

if __name__ == "__main__":
    if len(sys.argv) != 3:
        sys.stderr.write("Usage: ./emlToMbox.py input outbox.mbox\n")
        sys.exit(1)
    sys.exit( main( sys.argv ) )

Thanks, it saved my skin yesterday. I was getting some errors related to Print in Python 3.10 but all you need to do is add ( and ) to it, like:
print "Input is: " + infile_name
to
print ("Input is: " + infile_name)

fantastic! just needed the py3 fixes mentioned above by erebar.

Excellent @erebar! I'm honestly a bit surprised it worked under Python3 with only those minor changes. Great to know.

Thank you @kadin2048, this has been very useful when migrating mail providers!

With @kadin2048's blessing, I forked this into a repository (rather than a Gist), so I could combine it with an existing mbox de-duplicator (Jachimo/mbox_dedupe):

https://github.com/Jachimo/emlToMbox

Pull requests welcome.

@kadin2048 Please add a license comment so that we can use and modify your excellent work! Without a license, this code is illegal to reproduce (copy), distribute, or create derivative works from (modify).

The MIT license is pretty good, and I put your name on it for you. Simply copy and paste the following to the top of the file:

# Copyright © 2023 Kadin2048
#
# Permission is hereby granted, free of charge, to any person obtaining a copy of this software and associated documentation files (the “Software”), to deal in the Software without restriction, including without limitation the rights to use, copy, modify, merge, publish, distribute, sublicense, and/or sell copies of the Software, and to permit persons to whom the Software is furnished to do so, subject to the following conditions:
#
# The above copyright notice and this permission notice shall be included in all copies or substantial portions of the Software.
#
# THE SOFTWARE IS PROVIDED “AS IS”, WITHOUT WARRANTY OF ANY KIND, EXPRESS OR IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY, FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM, OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE SOFTWARE.

Alternatively, you could choose a license for yourself.

This worked great. Thank you! 🙏