Route by the recipient's real mail server. Not a guess from the domain string.
Vanity domains hide who’s really behind the mailbox. sendrules checks the recipient’s live mail server on the way in and routes on the answer — so acquisitions, migrations, and rebrands don’t quietly become bounce spikes.
The domain is the same. The mail server is not.
Your best customer’s business acquired their parent’s Google Workspace instance last month. You’re still routing their email as if it’s the old on-prem Exchange — a lane tuned for corporate policy servers, not for Google’s consumer-grade filters. The bounce rate on that domain went up, and the ESP dashboard says “delivery failed” with no useful reason attached. Nobody at your end was told the mail moved.
Ask the mail server. Believe the answer.
For every recipient, sendrules resolves the domain’s mail server before the send, matches it against a maintained classification of mailbox providers, and picks the provider and shaping profile for the lane the recipient is actually on. When Acme moved to Google Workspace, the routing changed with them — the first message after the migration went out through your Google-tuned lane, not the corporate one.
Why domain-string routing breaks.
Live mail-server classification, per recipient.
sendrules looks up the recipient domain’s active mail server on the way in, checks it against a maintained list of mailbox providers (Google Workspace, Microsoft 365, and every other lane we recognise), caches the answer with a short and honest time-to-live, and re-checks when the cache expires. The classification lands on the route log so a bounce three weeks later still has “this went to Google” attached — even if the mail moved again in between.
The classification then drives everything else: the provider selected under your rules, the shaping profile applied to the send, the postmaster view that surfaces the reputation, and the DMARC lane the report will land against.
Acme moves to Google Workspace on a Wednesday.
Under domain-string routing, Wednesday’s send to every recipient at Acme goes through the corporate lane you set up for them a year ago. The first tranche hits Google’s consumer-grade filters, gets treated as unrecognised bulk mail, and most of it lands in the promotions tab or the spam folder. The bounces trickle back over the following hours. Nobody knows the classification is stale — the ESP dashboard says “delivery failed” and stops there.
Under live mail-server classification, the Wednesday send sees Acme’s new mail server on the way through, routes to your Google-tuned provider, uses the shaping profile appropriate for that lane, and lands with the same acceptance rate as every other Google Workspace recipient in the send. The route log shows the classification changed and when. Nobody had to know the deal happened.