1. It appears that all emails are stored in this folder on the server:
Under this i see numbered sub-dirs, which look like 'userid' of the user. And messages are apparently stored under it.

Is this correct?

2. If so, i can use unix command: du -kh on a user folder to determine disk space used by a user.(are attachments inside this too?)
$ du -kh /zimbra/store/0/14
96 K /zimbra/store/0/14

Is this OK? I am just trying to understand how zimbra is able to calculate disk space used by a user.

3. Tried looking at the java source, but cannot determine, how zimbra is re-calculating disk space everytime a mail is sent etc. Any pointers to this?

Any information on the above will be very useful. Especially related to how zimbra really 'calculates' the disk space used by a user (i know of zimprov script, but that only gives me the space, how did it calculate it?).

Thanks for your help.