In my ISPConfig 3 installation, I compared my log with the output from Perfect server guide. I realize that this line is missing : Configuring SASL Configuring PAM Is it important? I check it with grep and found cyrus-sasl and pam, so I conclude it was installed on my server. Now the ISPConfig 3 server is running ok. But sometimes the POP3 and IMAP server suddenly become offline. Is it because the problem above? When it happens, the saslauthd service stopped by itself, but the dovecot and postfix still running. Usually I abort postfix , then restart the dovecot, postfix and saslauthd. After that the server is OK again. Just for info, I use a VPS, so I don't know for sure what is installed in the OS in the first place. For example, cron is not installed in the first place.