In the VPS I'm working on, Plesk is installed. By default, Plesk Server Administrator (PSA) uses Qmail mail server.
I first stopped Qmail and tried to start postfix but the same problem has occured. I then stopped psa daemon and
still got the same problem. As I said before, what makes it difficult for me to figure out the problem is that nothing is
reported in the logs. Mail log /var/log/maillog is even empty and doesn't contain any line.