Manual (file system level) message deletion

[absolutely_free at libero.it]

Hi,I use Dovecot + Postfix on CentOS server. I use maildir.
What happen if I manually delete some messages (eg. the ones older than 6 months) for every use?I think that dovecot's indexes will be wrong... is this a potential performance issue?If so, how can I "rebuild" index file for every user?
Thank you