Setting intervals for local (mbox) spool mailboxes does not work. They always get set back to 10 minutes. Reproducible: Always I found the same bug on debian bugzilla (http://bugs.debian.org/cgi-bin/bugreport.cgi?bug=572342) and tested the provided patch in my local overlay (just downloaded the patch into the files dir and added it to the PATCHES list in the ebuild) and it works.