Goals for next minor release (0.4.1):
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* Add a lot more tests (see top of test_archivemail.py)
* Check the sizes of the original mailbox before reading & just before
  overwriting. They should not have changed - otherwise somebody else is
  writing to it.

Goals for next major release (0.5.0):
-------------------------------------
* 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 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?
* Test for missing compression programs 
  - is this a waste of time?
* IMAP support
  - is this outside our scope?