dovecot 2.1.1

I access my IMAP folders through SquirrelMail

as a result of 'part copying' (probably through web client ? not sure), that is copying/moving some emails, later, the rest, I have a sub folder where I have maybe 30 emails from 2012 appear at the very end like so

2013/2012/2010/.../2006/2005/2012

how can I re-sort ?

thanks

Voytek