How to convert Maildir folders into mbox files

Brian Zammit writes ” There are a number of utilities to convert from mbox to Maildir, but only one working utility to convert from Maildir to mbox. Qmail itself comes with the working Maildir to mbox conversion utility, intuitively named maildir2mbox. However, it is limited when working with non-standard Maildir’s. At the moment the process of converting subdirectories of Maildir’s for a large number of users is very time consuming and clumsy at best.”