On Thu, Oct 12, 2023 at 02:02:55AM +0200, Daniel Ryšlink via Postfix-users
wrote:
> It's generally very useful to set up a specific transport for "sensitive"
> domains like gmail.com with specific policy (throttling outgoing message
> rate, etc).
>
> However, since more and more hosted domains use mail services for their
> domains, it would be useful to be able to select apropriate transport not
> according to domain or recipient e-mail, but according to the IPs address of
> the server specified by the MX record of the domain.
>
> Could it be achieved via Postfix configuration somehow? The transport table
> format does not seem to support this.
Sadly, no. Transport selection and scheduling happens in the queue
manager, which does not know which MX hosts an SMTP delivery might later
resolver to.
And even if it were MX-aware, what to when:
- Some MX host IP addresses are associated with one transport choice and
some with another?
- The actual MX hosts that are up and going to be used are not the
highest priority ones?
- The IP addresses that have not saturated their concurrency are not
the ones the SMTP delivery agent ultimately chooses when delivery
happens.
- ...
Adding Postfix support for IP-based concurrency limits across separate
nexthop domains would require a radical redesign of the queue manager
and/or SMTP delivery agents. So not likely any time soon.
Once a particular IP address is selected, the delivery would have to go
back into some sort of queue, freeing up the delivery agent that
performed the MX and address lookups, and that secondary queue would
have to implement concurrency limits by IP and pass the envelope back
for actual delivery. Something along the lines a 2-stage SMTP delivery
agent, with a handoff to a second-stage queue along the way.
--
Viktor.
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