speed problem

Hi,
I have one problem. mails are sent very fast and this cause delivery problems with some address emails (libero) that cause 451 error – slow down.

I have about 1000 emails to send everyday and I set 300 emails at hour.
So 25 emails all together are sent every 5 minutes!
But I need to send an email every two seconds and not all the block

How can I do?

In “send detail” I have this:
Sending an email is taking more than 1 second, rather slow. Read more. Average time to send an email: 47.60 seconds
Max speed: 75.63 emails per hour
Max mean time measured: 294.13 seconds
Min mean time measured: 0.29 seconds
Total email in the sample: 100
Runs in the sample: 100
Runs prematurely interrupted: 0.00%