postfix multiple instances on same machine using concrete IP for sending

[the_bat]

VPS with mail servers - each have it's own public IP:

mail.mydomain.com
mail.mailing1.mydomain.com
mail.mailing2.mydomain.com
mail.mailing3.mydomain.com

Send/receive emails works fine but

Problem:

When I sent test email from mail.mydomain.com to my gmail account I saw the strange header there

Received: from mail.mydomain.com (mailing3.mydomain.com. [public IP of mailing3 server])

No mather from which server I try to send email there is only used mailing3.mydomain.com public IP

It seams like postfix use all servers as pool not as separate instances

Question:

How to tell to the each instance of postfix to send email via concrete public IP

mail.mydomain.com -> public IP of mail.mydomain.com

mail.mailing1.mydomain.com -> public IP of mail.mailing1.mydomain.com

and so on

?