Adding/increasing sending limits for Qmail

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/concurrencyremote

The 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.