Archiving old messages from multiple nnml folders

[Giorgos Keramidas]

Hi everyone,

I have several nnml folders with messages from 2007-01-01.  Now I want
to archive old messages, e.g. by moving all read messages from all my
folders to 'yearly' archives like:

    nnml:mail.archive.2007
    nnml:mail.archive.2008
    nnml:mail.archive.2009

Is there an easy way of iterating through all nnml folders and archiving
the old and read messages in nnml folders like these?  I *think* what I
am looking for is a custom expiry target for nnml groups, but I am not
sure how to trigger an expiry run for *all* my nnml folders.

How would you go about doing something like this?