Deliverability is a reputation, and you are the one earning it.

What mailbox providers actually score.

Gmail, Outlook, Yahoo and the rest each keep a private record of your mail. The inputs are broadly the same everywhere, and the useful question about each one is whether it belongs to you or to the service sending on your behalf.

Two things follow. A new domain has no reputation, good or bad, so its first weeks are judged mostly on authentication and content, and on the shared IPs it borrows. And a reputation once damaged is repaired the same way it was built: by sending wanted mail to people who open it, for long enough that the history changes.

The complaint threshold that matters.

Only one major provider publishes a number. Google’s email sender guidelines (read on 13 September 2026) say to keep the spam rate reported in Postmaster Tools below 0.10%, and never to reach 0.30%. Below one complaint in a thousand you are fine. Between one and three in a thousand you are being watched. At three, Gmail starts filtering, and it takes weeks of clean sending to come back. Senders of more than 5,000 messages a day to Gmail must also authenticate with SPF and DKIM, publish a DMARC record (a policy of p=none is enough), and support one-click unsubscribe on marketing mail; the guidelines are worth reading in full once, because Yahoo’s match them closely.

A complaint is not an unsubscribe. An unsubscribe is a request you honour and nobody scores you for; a complaint is the recipient telling their provider your mail is unwanted, and it counts against the domain whether or not you would have removed them anyway. The practical conclusion is that the unsubscribe link must be easier to find than the spam button.

Warming a new domain on shared infrastructure.

Notix sends from shared IPs. There is no dedicated-IP option, and for most senders that is the better deal: the pool already has a history with every major provider, and the provider polices who sends from it, which is what the automatic suppression below is for. A dedicated IP starts with no reputation at all, has to be warmed by you over weeks, and loses ground every week you send too little from it. It earns its keep somewhere north of a hundred thousand steady messages a month.

What you still warm is the domain. In the first two weeks, send to the recipients most likely to open: recent sign-ups, active customers, anyone who asked for the mail this month. Keep the volume modest on day one and grow it steadily rather than moving an entire list across at once. Let transactional mail run throughout; receipts and codes are the most wanted mail you have and they build the domain’s record. Put marketing on its own subdomain so a bad campaign cannot drag password resets down with it; each subdomain you verify gets its own DKIM key. Watch bounces and complaints daily and slow down when either moves. Notix does not automate that ramp; the schedule is a decision only you can make.

When it goes wrong.

Deliverability problems announce themselves in one of a few ways, and each points somewhere specific.

The webhook events for delivered, bounced and complained mail are the raw material for all of this; the transactional email API emits them per message, and the dashboard shows the rates per domain.