Resolution

Ttry to connect your Plesk server as below (YOUR-PLESK-IP should be replaced with your real IP address):

Your server hostname should be reported instead of 'domain.tld' in SMTP greeting (qmail takes it from /var/qmail/control/me file). If you see another (non qmail greeting), it means that SMTP port is listen by some another software and Plesk SMTP service does not work because of this.

Which process is listening on 25 port can be found using the `/usr/sbin/lsof -i | grep -i smtp` command. If this process is xinetd you have to check if any other SMTP service is configured except for psa_smtp. It can be found using `grep smtp /etc/xinetd.d/*`.

If you find that 25 port is listened by 'sendmail' or some other process, you need to stop this process, disable it be started and make sure that qmail service is configured properly in xinetd (/etc/xinetd.d/smtp_psa and /etc/xinetd.d/smtps_psa) or inetd on FreeBSD (/etc/inetd.conf).