Hi, I have (Debian Wheezy/Sid) ISPConfig 3.0.5.4p8. I've noticed in mail queque such messages: (temporary failure. Command output: /usr/bin/maildrop: Unable to create a dot-lock at /var/vmail/[subdomain]/[user]/1847.0.[hostname].) But this user is deleted. There are few more such messages regarding deleted accounts. I think this started when I upgraded Debian from 6 to 7.
Thats ok, the message is just a bit misleading. It means that maildrop rejected the email because the account does not exist anymore on your server.
This means that 200 people sent emails to addresses that dont exist anymore and that these got rejected suucessfully. so nothing unusual. Postfix manages its queue on its own, so no need for any manual actions.