While this is fine for a standalone program, for scalability, you may wish to consider the Mail::Box family of modules. It's being actively maintained (in fact, the latest release was yesterday as I write this), and would probably have reduced your program to three lines.

-- Randal L. Schwartz, Perl hacker
Be sure to read my standard disclaimer if this is a reply.

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update: I may have posted a bit too soon. It appears you are translating from a non-standard message format that might not be supported by standard tools, in which case most of your program would not go away. My apologies. However, dealing with the Mbox format on the output could still benefit from using the module, so that others can reuse this in places that aren't using Mbox. I had never heard of MDaemon before, and didn't realize the unstandard the message format.