How it will work?
The server will rotate the IP with each sent email.
If you will send 10 emails and your 5 IPs are in the use in rotation,
1st email via 1st IP
2nd email via 2nd IP
3rd email via 3rd IP
6th email via again 1st IP
and so on..
Why we needed multiple IPs?
Whenever you will send an email, it will deliver with the help of a static IP address.
Each server comes with a single IP (default IP) address.
Hence we can not send a large number of emails with the single IP since the ISPs like Gmail, Yahoo, Outlook, etc will check each and every detail and email speed per minute and per hour from a specific IP is one of them.
If you will send too many emails from a single IP within a certain period then you will get block yourself with this kind of bulk mailing.
So in order to send bulk emails, we will need multiple IPs.
You can purchase a few additional IPs based on your server setup plan.