$ man acs-deliverability-routing

ACS / Deliverability Routing

Knowing which mailbox infrastructure lands where, and routing every send accordingly. Deliverability treated as a routed system with measured paths, not a prayer you say before hitting launch.

by Shawn Tenam

the same domain can be poison on one path and clean on another. I run sending domains that wall at Google on reputation, bouncing at 33%, while the identical domains land at Microsoft inboxes at a 1% bounce rate. one sending stack, two completely different outcomes, decided entirely by the receiving provider. if you don't know your landing rates per provider, you're averaging a great path and a terrible path into one mediocre number and torching domains to learn it. routing fixes this: classify every contact by mail provider before sending, and only send down paths you've measured. Azure Communication Services (ACS) is one of those paths, sending infrastructure you control at the API level instead of renting mailbox seats.

every send in my stack passes through a routing gate. an MX classification script tags each contact with their email provider, and the send logic routes on that tag: contacts on providers where my domains land go through the ACS pipeline, contacts on providers where those domains wall get held back or routed to a different channel entirely. the gate is code, so it can't be skipped by enthusiasm. I learned the split by measuring bounces per provider, not per campaign. once the paths are mapped, deliverability stops being a mystery that strikes at random and becomes a routing table you maintain.