Each of the various mail user agents uses different lock mechanisms. And some mail user agents produce messages that don't look like locked mailbox problems at all, e.g. Netscape Mail can report that it cannot process 'From' lines and that the recognition mode needs to be reconfigured.
Check for the lock file web3:/var/mail/username.lock, also check for running 'popper' processes and kill them first normally to allow them time to remove their lock file, and then with -9. If there is no popper process then simply remove the lockfile.
This is usually because the user *really* is running another instance of a MUA either on the same machine or on another one.
Ensure no MUA's active for the username, kill all 'popper' processes on the mailserver, remove and /var/mail/<username.lock>, move the mailfile <mailserver>:/var/mail/<username> out of the way and also <mailserver>:/var/mail/.<username>.pop. Then send a test message and check it is readable with Netscape.
$Header: /home/ct/serco/ahuxley/docs/html/serco-faq/new/network-services/mail/RCS/problem-mailbox-locked.html,v 1.1 1997/06/13 12:34:34 ahuxley Exp $