the log output you sent seems to be not giving any clues.
the second one is showing that some clients were trying to logon or connect your mail server but they failed due to wrong authentication.
the first one ( log file doesn't exist), please try to figure out whether the log exists on your file system. Do manually "ls -al /var/log/maillog". if it doesn't exist, try to restart postfix service, hopefully it will create it, if still, it doesn't , create it manually "touch /var/log/maillog" and restart postfix service then see what happens
