postfix/main.cf: missing proxy: prefix on some mysql lookup tables
because some of the mysql lookup tables are not prefixed with "proxy:" in /etc/postfix/main.cf there can are many (sleeping) mysql processes, some idling almost forever, they will block db access if the server reaches max_connections.
the recommended configuration is to use postfix proxymap servers for mysql lookup tables, sharing connections, it will reduce mysql connections to a minimum.
debian buster - ISPConfig 3.1.15p2
add proxy: to all mysql lookup tables, and adjust proxy_read_maps
after the change the processes which sleep at most, are the two from amavis (one for IPv4, one for IPv6), they sleep until there is a mail incoming, all other connections seem to be terminated within less than a minute.