Run the following commands to get it done(Increase the Value according to your need)
echo "120" > /var/qmail/control/timeoutremote echo "120" > /var/qmail/control/timeoutsmtpd echo "40" > /var/qmail/control/concurrencyremoteThe default timeouts are set to 20 minutes, or 1200 seconds. So if the mail server on the other end is not responding, the outgoing email stays in qmail's remote queue for 20 minutes before it fails. By default, there are 20 outgoing slots for email so this can backup quickly. These settings also increase the number of emails which can be sent at one time to 40. Then restart the qmail and you are done.