Goals for next minor release 
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* When you get a file-not-found in the 6th mailbox of 10, it aborts the whole
  run. Better to fail gracefully and keep going.
* Think about the best way to specify the names of archives created with
  possibly an --archive-name option.
* Add more tests (see top of test_archivemail.py)
* We need some better checking to see if we are really looking at a valid
  mbox-format mailbox.
* Lock any original .gz files 
  - is this necessary?
* Check for symlink attacks for tempfiles (although we don't use /var/tmp)

Longer Term goals:
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* Add an option to not cut threads.
* Add MMDF mailbox support
* Add Babyl mailbox support
* Add option to archive depending on mailbox size threshold 
  - is this a good idea?
* Add option to archive depending on number of messages
  - is this a good idea?