A problem in work by Qmail , Sending many mails in one moments in large scale , always I had a question about it , after research and change configure I found 3 step to solve it .
Please Check qmail for exists under files in (/var/qmail/control):

• timeoutremote
• timeoutsmtpd
• concurrencyremote

ls /var/qmail/control | egrep ‘concurrencyremote|timeoutsmtpd|timeoutremote’
After it following commands :
echo “400″ > /var/qmail/control/concurrencyremote
echo “100″ > /var/qmail/control/timeoutremote
echo “100″ > /var/qmail/control/timeoutsmtpd
Note : Default timeout after fail send mail in queue is 20 minutes , (It’s high)

Now restart QMail .
Be careful increase top numbers related by ram on server .
I Change it on my server by under configure and answered good :
Ram : 6 GB
CPU : Xeon 3.8

Mail should now begin to process faster on your Dedicated Server

After Change it I can send 6000 Mails in One hour.