Sending eMails via a server with Plesk and Debian can cause some delay. This is due to the mail server trying to get information regarding your IP, which doesn't always succeed.
Changing this reaction requires a little modification in the file /etc/inetd.conf.
Open the file with a text editor, browse through the file until you reach the end.
Change the lines with "smtp" and "smtps" as follows:
append the option "-Rt0" to the line "/var/qmail/bin/tcp-env".
smtp stream tcp nowait.1000 root /var/qmail/bin/tcp-env tcp-env /usr/bin/env
smtp stream tcp nowait.1000 root /var/qmail/bin/tcp-env -Rt0 tcp-env /usr/bin/env
Now building up the connection should work definitely faster.