Postfix - Mail system is down

Solution 1:

Is postfix actually down? If you keep seeing messages about postfix master starting, then presumably it's also stopping in between? (whether it logs anything or not).

Are you running mailq? That would produce that warning about "Mail system is down".

If there's a problem with your virtual user setup, then you'd like to get some info on what's going on in the mysql queries. Maybe capture some DB traffic with tcpdump and run it through Percona's pt-query-digest and related tools. Or maybe just log all queries to the slow query log (ie threshold execution time = 0).