I have this problem too, and when I try to move/delete more than 80 mails I got an error about the script don't responding
Code:
Script : http://zimbra.ac-fr.com/zimbra/js/Startup1_1_all.js.zgz?v=090830153842:5493
I use the Release 6.0.0_GA_1802.DEBIAN4.0 DEBIAN4.0_64 FOSS edition.
This will be nice to fix this bug.
At this time I don't think this version is usable in production mode.
Sam.