I restarted postfix and got the following message:
Nov 23 06:26:44 sentinel postfix/virtual: fatal: mysql:/etc/postfix/mysql-virtual_mailboxes.cf: proxy map is not allowed for security sensitive data
I also note that the logs report:
Nov 23 06:26:45 sentinel postfix/master: warning: process /usr/lib/postfix/virtual pid 26916 exit status 1
Nov 23 06:26:45 sentinel postfix/master: warning: /usr/lib/postfix/virtual: bad command startup -- throttling
I've just had 40 odd emails arrive too. It doesn't appear to have stopped mail being delivered, but I'm still concerned about the error.