Imagine you write the perfect email: the ideal subject line, the exact angle, the right call to action. And nobody replies. Not because the message is bad, but because it never even reached the inbox. It landed in spam, or worse, the servers discarded it before delivering it.
Deliverability is the least flashy part of email marketing and, at the same time, the most decisive. It is the invisible foundation everything else is built on: the best copy and the best list are worth nothing if your message does not arrive.
In this guide we are going to cover it in full and without jargon: what it is exactly, how Gmail and Outlook decide whether you get in, how to authenticate your domain, why you need dedicated domains, how to warm them up, how to keep your list clean, what to monitor, and what to do if you are already landing in spam.
What is deliverability (and how does it differ from delivery)?
Many people believe that if the email "was sent", that is it. It is not so. There are two concepts worth separating.
Delivery vs deliverability
The delivery rate measures how many emails did not bounce, that is, that the recipient's server accepted them. Deliverability measures something far more important: of those that were accepted, how many reached the inbox and not spam. You can have 99% delivery and still have 80% end up in the junk folder. Your sending dashboard would say everything is fine while your campaign dies.
Inbox, promotions, and spam
Not all "not the primary inbox" are equal. Landing in Gmail's Promotions tab is bad for prospecting (almost nobody looks at it), but it is not spam. Landing in spam is the death sentence. The goal in outbound is always the primary inbox, because it is the only one your prospect truly checks.
How Gmail and Outlook decide whether you arrive
The providers do not have a public rule, but they do have a clear principle: they protect their users from what they do not want to receive. To decide where to place you, they look above all at your sending reputation, which is built from three ingredients: who you are (authentication), how you behave (volume, consistency, bounces, complaints), and how people react to your emails (whether they open, reply, mark as spam, or delete without opening).
The practical consequence is huge: deliverability is not "configured", it is earned over time and can be lost in one bad week. Everything that follows are the levers to build and protect that reputation.
Engagement is king
If you had to keep just one factor, it would be this: how people react to your emails. For Gmail and Outlook, the best proof that you deserve the inbox is that your recipients open, read, and above all, reply. And the worst possible signal is that they mark you as spam or delete you without opening.
This has an implication many people overlook: deliverability and message quality are not separate problems. A relevant email, aimed at someone who fits, generates replies, and those replies improve your reputation, which makes your next emails arrive better. It is a virtuous circle. The reverse, a generic message sent to a bad list, generates silence and complaints, and that sinks the deliverability of everything else. That is why segmenting well and writing relevant copy is not just "better marketing": it is, literally, better deliverability.
Authentication: SPF, DKIM, and DMARC
They are the way Gmail or Outlook check that you are who you say you are. Since 2024, Google and Yahoo require them from anyone sending in volume. Without them, you start half the match already lost before writing a word.
SPF
It says which servers are allowed to send emails on behalf of your domain. If you send from a server that is not on the list, they suspect you.
DKIM
It cryptographically signs each email to prove it has not been tampered with along the way and that it really comes from your domain.
DMARC
It tells the provider what to do if SPF or DKIM fail (nothing, quarantine, or reject) and, in addition, sends you reports of who is using your domain. Configuring all three is an afternoon's work and protects all your future campaigns.
Use dedicated domains and mailboxes
Never do cold prospecting from your main domain. If a streak of complaints damages the reputation of "yourcompany.com", your emails to customers and your invoices will suffer too. It is a risk not worth taking.
The sensible move is to register one or several similar secondary domains (for example, "yourcompany-mail.com" or "try-yourcompany.com") and send from there. If one gets damaged, you retire it and keep operating without touching your main reputation. Within each domain, spread the volume across two or three mailboxes and do not squeeze any of them: many mailboxes sending little beat one sending a lot.
Warm up, week by week
A freshly created mailbox that sends hundreds of emails on the first day is a huge red flag. Mailboxes, like people, earn trust little by little. A reasonable warm up looks like this:
- Week 1: 10 to 20 sends a day, ideally to contacts who will probably reply (customers, acquaintances), to generate positive signals.
- Weeks 2 to 3: raise it gradually to 40 to 60 a day, maintaining replies and without abrupt spikes.
- Week 4 onward: reach your target volume, without exceeding around 40 to 50 cold emails per mailbox per day.
It is slow and boring, and it is exactly what separates a healthy account from a burned one. There are "warm up" tools that automate part of the process by exchanging emails between real accounts; they help, but they do not replace patience.
List hygiene: the silent factor
Bounces and complaints are poison for your reputation. Each invalid address tells the provider that you do not look after your data, and each spam complaint weighs like a slab.
Thresholds worth not exceeding: bounces below 2% to 3%, spam complaints below 0.1% (one complaint per thousand sends). Go beyond that and your inbox delivery drops for all your emails, not just that campaign.
Verify each address before sending with a validation tool and remove the ones that bounce after the first attempt. The rule is simple: less volume and cleaner always beats more volume and dirty. A purchased, unverified list is the fastest way to burn a new domain in days.
It goes a step further: segment by engagement too. If you have gone several touches without a contact opening or replying, take them out of the sequence. Continuing to write to someone who ignores you is not only useless, it sends Gmail the signal that your emails are not interesting, and that hurts the delivery of the rest. Looking after the list is not just cleaning bounces: it is stopping pushing where there is no sign of life.
Content also scores
Deliverability does not end with the technical side: the email itself influences it. Too many links, heavy images, attachments, or words like "free", "offer", and "guaranteed" raise your spam score. Generic link shorteners also raise suspicion.
A plain text email, brief, with one link at most and no images, arrives much better than an email laid out like a newsletter. In cold prospecting, the simple not only converts more: it also gets in better. If your message looks like one person writing to another, the filters treat it as such.
How to monitor your deliverability
You cannot improve what you do not measure. These are the indicators and tools worth watching every week:
- DMARC reports: they tell you if someone is spoofing your domain and whether your authentication passes.
- Complaint and bounce rate: your early alarms; if they rise, slow down before it is too late.
- Blacklists: check with free tools whether your domains or IPs appear flagged.
- Postmaster tools: Google Postmaster Tools shows you your reputation in Gmail's eyes.
- Inbox tests: test sends to several accounts to see where you really land.
What to do if you are already landing in spam
If your reply rate has collapsed and you suspect deliverability, do not send more emails: that only worsens the damage. Follow a recovery plan:
- Stop and diagnose: review authentication, bounces, complaints, and blacklists to locate the cause.
- Cut volume drastically: return to a trickle of sends to contacts who reply, as in a warm up.
- Clean the list thoroughly: remove bounces, dubious addresses, and anything unverified.
- If the domain is badly damaged, retire it: sometimes it pays off more to start with a new one than to rescue a burned one.
Recovering reputation takes weeks, not days. That is why prevention costs much less than cure.
Mistakes that burn your domains
- Prospecting from the main domain: you put your customer email and invoices at risk.
- Skipping warm up: high volume from day one equals an immediate red flag.
- Purchased, unverified lists: bounces and complaints that sink reputation in hours.
- Ignoring opt outs: continuing to write to someone who asked to stop receiving triggers complaints.
- The same identical message to thousands: the filters detect the pattern; vary and personalize.
Frequently asked questions
How many cold emails can I send per day per mailbox?
As a prudent reference, 40 to 50 per mailbox per day once warmed up. It is better to spread the volume across several mailboxes than to squeeze a single one.
Do I need a warm up tool?
It helps speed up and maintain reputation, but it is not essential if you warm up by hand with discipline. What is not optional is the warm up itself.
How do I know if I am landing in spam?
The clearest sign is a sharp drop in replies without changing the message. Confirm it with inbox tests to several accounts and with Google Postmaster Tools.
Does deliverability depend on copy or on the technical side?
On both. The technical side (authentication, domains, warm up) gives you access to the inbox; the content and the recipients' behavior decide whether you stay in it.
How often should I review it?
Weekly as a routine. Deliverability is a living state: it is maintained, not solved once and for all.
How long does a new domain take to warm up?
Between two and four weeks to reach a reasonable working volume. Being in a hurry here is the mistake that burns the most domains: what you save in days you pay in months of bad reputation.
Is it worth asking to be added to contacts?
A lot. A recipient replying to you or saving you as a contact is one of the strongest positive signals for the provider. That is why a first batch of sends to people who do reply accelerates the warm up so much.
In short
Deliverability is the invisible foundation of all your email marketing. Authenticate your domain, prospect from dedicated domains, warm them up patiently, keep the list impeccable, write like a person, and watch your indicators every week.
Fix the foundations and maintain them, and suddenly everything takes off: the same campaigns that used to go unnoticed start getting replies, because someone is finally reading them. If you would rather not fight with the technical side, at Desorbitante we operate all that infrastructure for you so your messages arrive, always.
