When maildir_use_size_file, it creates the maildirsize file with the
quota, but it never recalculates this file as per spec. If the quota value
changes in the transport, the maildirsize file is never updated to reflect
this.
------------------------------
http://inter7.com/courierimap/README.maildirquota.html
Calculating the quota for a Maildir++
.....
4. If the numbers we got indicated that the Maidlir++ is over quota,
some additional logic is in order: if we did not recalculate maildirsize,
if the numbers in maildirsize indicated that we are over quota, then if
maildirsize was more than one line long, or if the timestamp on
maildirsize indicated that it's at least 15 minutes old, throw out the
totals, and recalculate maildirsize from scratch.
