Yes it does
max_connections = 250
max_user_connections = 0
A value of 0 means "no limit", so it'll go up to 250 if needed.
I will try 500, see how that goes, but still, maybe we should look for another way to prevent such an amount of connections being made to the MySQL server for checking some mail rules/filters. Since people will run websites on the servers as well, who need the connections.
Maybe limit the max concurrent connections postfix is allowed to make (so split up the user ispconfig to (f.e.) ispconfig_adm and ispconfig_mail. Set the mail user to only use X amount of connections, so the server admin nor the website users won't be affected when a server gets hammered by spammers.