Postfix limits for multiple SMTP possible?

Question

postfix

Task:
We receive letters from the site that go to postfix.
He, in turn, looks at the sender's email or the subject of the letter, and if one or the other matches the condition, then he sends it to the desired smtp.

According to the header, I managed to configure catching letters and sending them to the specified smtp server. But I did not find any mention anywhere that it was possible to set limits for each smtp by the number of letters per hour.

Everywhere it is said that limits can be set only for each domain.
But the trouble is that this implies that you have several sites and they are running under different users and each user has their own domain, and under it there are already limits and smtp.

But I have 1 site, and even if I even made php change the sender's email depending on the subject of the letter, then I can't force postfix to bind the sender to smtp or a domain.
My sender is always root. Yes, I can match root'a with an email address, but there is no benefit from this.

Prompt how it is possible to configure limits for each smtp?