Goal

I'm looking for a solution to limit rate of e-mails sent to specific e-mail operator (several Top-Level domains, all of them I know) on hourly basis. I also need some way to control the rate on per-mail basis, to allow bigger amount of e-mails sent by postfix during each hour. Limit is global for all addresses maintained by this operator, reseting each full hour.

What I've done so far

I use postfix after_queue custom filter, looking for our custom header indicating priority. Number of mails accepted by filter are kept outside script, within PostgreSQL database. Mails fitting current timeframe quota are passed to postfix as accepted, mails outside quota are deferred using tempfail return code.

SMTP relay used to send e-mails to this specific operator is separate virtual machine, used only for limited e-mails and bounces from the operator. Custom filter efficiency is good enough according to requirements.

Problem

Basically, there are two problems:

1. I couldn't find any way of prioritizing e-mail messages in postfix queue. It would be preferable if I could use our custom header as priority indicator or add another priority mark to e-mail message and be guaranteed it will be filtered and sent before non-priority messages. I thought about using multiple instances (postmulti), but priorities are required to be relative, numeric – those are used as quota modifier for outgoint mail. Basically: email with priority N uses 1/N of quota used by email with no priority. Prioritizing is nice-to-have feature, so we can go away without it if timeframed quotas work correctly.
2. Due to e-mails going back to queue after applying filter we are not guaranteed e-mail will be really sent in timeframe it was accepted. Postfix queue seems impossible to steer and control real throughput.

My idea of solution

From after-queue custom filter, instead of returning message to postfix queue, just send it directly to destination SMTP using another tool and skipping postfix entirely. I don't know if there is any ready-to-use smtp client available on Linux to achieve such tasks. Also, we need to process bounce messages both synchronous (as response in SMTP communication) and asynchronous (as status e-mails sent to Return-Path address). We also need to control Return-Path for such tool to point the server e-mail is sent from.

As a last chance solution, the idea is to roll out own SMTP server or maybe modify sendmail/postfix source to our needs. It is overkill until no other option is available.

Question

What Linux tool can immediately (without queueing) relay e-mails to recipient SMTP server and provide valid bounce messages? Or, if there is better solution of this problem, what tools should I look at?