|I know this might be considered as stupid suggestion, but mailbox is in fact kind
of a text file.
Move all the files you want to export into separate folder, eg. "export". Then find the file
export.mbx in your Poco directory or subdirectory, rename it to export.txt. Then go to
Poco and delete the mailbox (if it will still appear on the mailbox pane).
All the info is stored in text format, although you might maybe find the email headers to
be excessive. But even this can be solved some way.
Note from Michael: You could use a script to strip the excess headers from the messages if you'd like
or you could use a modified form of the "Archive Old Mail" script that comes with Poco.
Poco already supports a method for saving individual messages as text files.