zimbra puts the account in maintenance mode while a backup is being done. (ie user 'a' will be locked when user 'a's' mail is being backed up.
to see who's mail is being backed up ..
run (as zimbra user)
zmbackupquery
should give something like
Label: full-20090xxxxxxxxxx
Type: full
Status: in progress
Started: Mon, 2009/04/27 01:00:58.126 EDT
Redo log sequence range: 1794 .. 1805
Currently backing up:
user@domain.com
Number of accounts: xxx
xxx out of xxx accounts have been backed up
hope this helps