The script at jooooooon/maildir2mbox was so slow I had to rewrite it, just because it baffled me to see a script with so horrible performance.
The original script was a bash script calling repeatedly a perl script, creating many subproccess and pipes; it uses 100% CPU just for nothing.
A simple fix would have been to rewrite the whole thing in perl to avoid calls to external script but I know nothing about perl so I made it in python. python2 has a bug with strptime and %z so I had to make it a python3 script.
Just cd into the directory with the maildir emails. And pipe the output of the script to your desired mbox file. For example:
cd /into/the/directory/with/my/emails/
./maildir2mbox.py > /tmp/test.mbox
This gives a mboxrd file. This is the variant of mbox file where every 'From ' line in the message is quoted (even if it is already quoted)
The current script has certainly bugs. It can't handle all the encodings of imported emails for example. It makes "heavy" use of memory too.
Side-effect: it rewrites the headers of each message in one single line
Another attempt: https://gist.github.com/2693582 This one uses the mailbox python module. Why bother to reinvent the wheel where thereis a module for it ? But it outputs mboxo only format and is slow. However it converts all mboxes in a given directory and its submail folders directly to /tmp