Email deliverability for B2B cold email: how inbox placement works and how to fix it

When you send an email, the receiving server first decides whether to accept it. If it does, the email counts as delivered, and that is the "delivery rate" most sending tools show. Then the provider decides where to put it: the primary inbox, a tab like Promotions or Other, or the spam folder. That second decision is inbox placement, and it is what people mean by deliverability.

A cold email campaign can show 99% delivered and still land half its mail in spam. Open rates will not tell you either, since Apple Mail Privacy Protection and security scanners open emails automatically. Reply rate per mailbox and seed tests are the honest signals.

Cold email has a harder time than newsletters. The recipient never signed up, so there is no engagement history to lean on, and the sending domains are new. Everything below is about earning trust from that starting point.

Gmail, Outlook, and Yahoo score each message on signals they keep about the sender and the content. They do not publish the weights, but the inputs are well understood.

Authentication

SPF lists the servers allowed to send for your domain, DKIM signs each message, and DMARC tells providers what to do when a message fails and whether the From domain lines up with the domain that passed. Missing or broken records get mail rejected or filtered before reputation even comes into it. Our email deliverability checker tests all three for any domain.

Domain and IP reputation

Providers remember how mail from your domain has been received. A new domain has no history, which is why it needs warmup and a slow ramp. With Google Workspace or Microsoft 365 mailboxes you share their IPs, so domain reputation carries most of the weight. On your own SMTP servers, IP reputation matters too.

Bounces and complaints

Sending to addresses that do not exist is one of the clearest spam signals there is, so unverified lists burn domains fast. Spam complaints count even more heavily. Google measures them in Postmaster Tools and wants them under 0.3%, ideally under 0.1%.

Engagement

Replies, moving a message out of spam, and adding a sender to contacts all help. Deleting without reading and marking as spam hurt. This is why targeting is a deliverability lever: people who have a reason to care reply, and replies protect the domain.

Volume and sending pattern

Sudden jumps in volume look like a compromised account or a spammer. Steady, modest volume per mailbox, spread across several domains, looks like a person.

Content

Links, images, tracking pixels, and link shorteners raise the odds of filtering, especially on a new domain. Spam trigger words matter less than people fear. A plain text first email with no links is the safest start.

Since February 2024 Google and Yahoo have published rules for everyone who sends to their users, with stricter rules for bulk senders (about 5,000 messages a day to one provider). Google began enforcing them more firmly in November 2025, with more non-compliant mail deferred or rejected outright. Microsoft added similar rules for Outlook.com, Hotmail, and Live addresses from May 5, 2025.

A cold email setup spread across many mailboxes sends far less than 5,000 a day to any one provider, so the bulk rules rarely apply on paper. We follow them anyway on every sending domain, with full SPF, DKIM, and DMARC and a plain opt-out line that we honor right away, because the filters reward the same behavior whatever your volume. Sources: Google's sender guidelines, Yahoo's sender best practices, and Microsoft's announcement, checked November 2026.

This is the order we set things up for a new client. The mailbox and domain buying side is covered in more depth in cold email infrastructure.

Start with the pattern. Whether the drop hit every mailbox or one domain, one provider or all of them, tells you more than any single score.

No single email deliverability platform does everything well. We pick a tool per job, and most of them are cheap or free.

Setting up domains takes a day. Keeping them healthy for a year is the real work, and it is the part most teams skip. We run deliverability as an ongoing service inside the outbound systems we build.

Every domain and mailbox is tracked

We keep a registry of every sending domain and mailbox: which provider set it up, where its DNS lives, when it was created, when warmup started, and when it first sent a campaign. Each mailbox moves through stages (purchased, warming, ramping, sending, resting, retired), and every change is logged with what it was before. Age counts from the first real send, so a domain bought six months ago that started sending last week is still treated as new.

Daily numbers per mailbox

Each day we record sends, bounces, replies, positive replies, and warmup placement for every mailbox, along with its settings that day. That is how we see one domain sliding while the rest hold steady, well before reply rates show it in the campaign view.

Pause rules

A mailbox that passes 2% bounces or 0.3% spam complaints is paused and rested. Volume moves to healthy mailboxes, and spare domains are warmed ahead of time so the volume has somewhere to go.

Warmup settings get tested

We run controlled tests on warmup settings, such as starting volume and daily ramp, with mailboxes split at random and stratified by domain, then compare placement, bounces, and replies between the groups before we change the default.

You own all of it

Domains, mailboxes, and the sending account are in your name. If we stop working together, everything keeps running.

A one-off email deliverability consulting audit makes sense when you already have a team running outbound and need someone to find the problem. If nobody owns sending day to day, an audit only resets the clock. OneGTM covers both: we diagnose what broke, fix it, and can keep running the sending afterward as part of a cold email or outbound build.

We built the email infrastructure for SuperDial alongside its inbound and outbound systems, and we run our own outbound on the same setup. More client work is on the case studies page.