Cannot assign dedicated IP to 1 user account for mail

Hello so we have here 2 accounts ( Example accounts ) both with different IP addresses. Ive created Mail users for both accounts but they still are using the server MAIN ip for mailing.

We want to be able to use a dedicated IP on the user account so ;

web1.com is using ip 1.1.1.1

web2.com is using 1.1.1.2

and so on.

So each user / Account has its own dedicated IP for sending email But this isnt working as i said it using same MAIN ip. How can we fix this