Converts a directory full of .eml files to a single Unix "mbox" file.
Accepts as input either an individual .eml file or a directory containing one or more .eml files.
The output mbox will be created if it doesn't already exist. If it exists, it will be appended to.
There is no checking for duplicates, so use caution.
If duplicate filtering is desired, it could be added to add_file_to_mbox().
Inspired by http://www.cosmicsoft.net/emlxconvert.html
Usage:
Directory mode: $ ./emlToMbox.py inputdir/ output.mbox
Single-file mode: $ ./emlToMbox.py input.eml output.mbox