[exim] rewrite message subject

All,

When email is targeted at certain mailboxes (mailing lists), I'd like to
rewrite the message subject to either prepend or append some text if the
subject does not already contain that text as a substring. This way,
mail that goes through the list will be stamped with something like
'[LISTNAME] original subject'.

Does anyone have any pointers on how I can most easily do this?

-- Dante