One of my users can't see the "empty trash" button in his "manage folders" menu. I've checked that he has messages in the trash folder. His quota is 95% full but as far as I know it has nothing to do with the problem.
Thank you very much.
I found out that having the option "Mark messages as deleted" enabled instead of "Move deleted messages to folder: ..." (Options -> Mail -> Settings) causes the "empty trash" to dissapear.
I hope this helps.