Deliverability is whether your email reaches the inbox, and it is earned every day, not set up once. This chapter explains what mailbox providers look at, how to watch your reputation with free tools, how to diagnose a drop step by step, and the daily habits that keep a sending domain healthy for years.
What decides where your email lands
When your message arrives at Gmail or Outlook, three kinds of evidence decide where it goes. First, identity: does authentication prove the sender is who they claim? Second, reputation: how have recipients reacted to mail from this domain and mailbox before? Third, the message itself: does it look like the kind of mail people want? Chapter 3 handled identity. This chapter is mostly about reputation, because that is where cold email wins or loses.
Authentication. SPF, DKIM and DMARC pass and align with the From domain. The ground floor. Chapter 3.
The number that matters most
Of all the signals, spam complaints carry the most weight. When a reader clicks "Report spam", they tell the provider directly that your mail is unwanted. A handful of these can outweigh many quiet deliveries.
For a small cold email program, those thresholds leave very little room. A few annoyed readers in a day can push a mailbox over the line. That is why everything earlier in this course, from tight targeting to short sequences, is also deliverability work. Relevant mail does not get reported.
Watching your reputation
You cannot log in to Gmail's filters, but several free tools show you what they see. Set these up once and check them on a schedule. A few minutes a week is enough when things are calm.
| Tool | What it shows | How often to check |
|---|---|---|
| Google Postmaster Tools | Spam rate, authentication and delivery errors for mail to Gmail users, per domain | Weekly, daily during a problem |
| DMARC reports | Which servers send as your domain and whether they pass | Weekly at first, then monthly |
| Seed inboxes | Where a test message lands: inbox, other tab, or spam, on accounts you own | Before each new campaign |
| Bounce messages | Why a message was refused, in the provider's own words | Daily, through your sending tool |
| Blocklist checks | Whether your domain appears on public blocklists | Monthly, or when bounces jump |
Postmaster Tools needs volume
Google only shows data once a domain sends enough mail to Gmail users. A small sender may see an empty dashboard. That is normal. Rely on seed inboxes, bounces and replies until the data appears.
Read your bounces
Bounce messages are the most direct feedback you get, and most people never read them. Each one carries a code and a short explanation from the receiving server. Learning to read them tells you whether the problem is the address, your setup, or your reputation. Exact wording varies by provider, so read the text, not just the number.
| Bounce says something like | What it usually means | What to do |
|---|---|---|
| User unknown, mailbox does not exist | Bad address | Remove it. Check how it got on the list. |
| Mailbox full, try later | Temporary problem on their side | Retry later. Remove if it repeats for weeks. |
| Authentication failed, does not meet required authentication level | SPF, DKIM or DMARC problem | Stop sending from that domain and fix the records. |
| Message rejected as spam, or policy reasons | Reputation or content problem | Pause, review recent lists and messages, slow down. |
| Rate limited, too many messages | Sending too fast | Lower the daily limit and spread sends over the day. |
Diagnose a drop, step by step
When replies dry up or test messages start landing in spam, resist the urge to change everything at once. Work through the causes in order, from the most basic to the most subtle. Most problems are found in the first two steps. Write down what you checked and what you found, so the next person who sees a drop does not start from zero.
Auth. Send a test to Gmail and open Show original. SPF, DKIM and DMARC should all say PASS.
Run a seed test properly
A seed test means sending your real message to a few inboxes you own and checking where it lands. It is the closest thing you have to seeing through the reader's eyes. Done carelessly, it tells you very little, so follow a few rules.
Use real accounts on the big providers
A personal Gmail, a Google Workspace address on another domain, an Outlook.com account and a Microsoft 365 address if you can.
Send the real message
Same mailbox, same template, same links and tracking settings as the campaign. A cleaned-up test proves nothing.
Do not interact first
Check placement before you open, reply or move anything, since your actions teach the filter.
Repeat before each campaign
Placement changes over time. Last month's clean test does not cover this month's new template.
Spread the risk
Even careful senders have bad weeks. The way to survive them is to make sure no single problem can stop everything. Use several sending domains, each with a few mailboxes, as Chapter 3 recommends. Assign each campaign to more than one mailbox. Keep your main company domain out of cold email entirely. Then when one domain struggles, you pause it and the others carry on, while your invoices and support replies keep arriving in your customers' inboxes as normal.
Recovering a damaged domain
Stop cold sending from it
Keep normal use and gentle warm-up going so the domain is not suddenly silent.
Fix the cause
Clean the list, repair records, remove the risky content. Recovery without a fix just repeats the problem.
Rest it
Give it a few weeks. Reputation improves as good signals replace bad ones.
Restart low
Resume at the bottom of the warm-up ramp with your best-fit contacts, and climb again slowly.
Know when to retire it
If a domain keeps struggling after a careful restart, retire it from cold email and move to a fresh, warmed one.
Content: what still matters
Content filters are less important than they once were, because reputation does most of the work. Still, some content habits send clear signals.
- Links. Each link is a risk, and link shorteners are a big one. Use none in the first email if you can, and at most one later, pointing to your own domain.
- Tracking. Open tracking adds a hidden image; click tracking rewrites links through another domain. For cold email, turn open tracking off and use click tracking only on a custom domain, if at all.
- Images and HTML. Plain text looks personal. Designed HTML looks like marketing.
- Attachments. Avoid them in cold email. Offer to send instead.
- Spammy phrases. Free, risk-free, act now, winner. Not because of a magic list, but because they come with the pushy tone readers report.
Unsubscribe links in cold email
Most cold programs send far below that threshold per domain, but the principle is sound for everyone: make it easy to opt out. A simple line such as "If this is not relevant, reply and I will not write again" works in a personal email. Laws in some regions require a clear opt-out in commercial email, as Chapter 13 explains. An easy exit lowers spam reports, which protects your reputation more than any other single habit.
Myths worth ignoring
- "A deliverability score of 10 out of 10 means you will land in the inbox." Test tools check your setup, not your reputation with real readers.
- "Dedicated IPs fix everything." On Google Workspace or Microsoft 365, the provider runs the servers. Your domain reputation is what you control.
- "Changing a few words beats the filter." Rewording spam keeps it spam. Fix the targeting instead.
- "High open rates mean good delivery." Opens are unreliable and Google says it does not track them. Watch replies and spam rate.
Daily habits that keep you in the inbox
0 of 6 checked. Answer replies the same day: Real back-and-forth is the strongest good signal you can earn.
When to pause sending
Sometimes the right move is to stop for a few days. Pause a mailbox when bounces suddenly rise, when spam complaints climb in Postmaster Tools, or when providers start rejecting your messages. Keep answering replies while you wait. Find the cause, fix the list or the setup, then restart slowly at a lower daily limit. Pushing on while the signals are bad only digs the hole deeper.
Doing this in FirstSales
FirstSales checks your sending domains' SPF, DKIM and DMARC records when you connect a mailbox, keeps each mailbox within its daily limit, removes bounced and opted-out contacts across campaigns, and shows bounces and replies per mailbox so you can spot a struggling one early. See the mailbox health view with sample data in the demo.
Key takeaways
- Inbox placement rests on identity, reputation and content, in that order of importance.
- Spam complaints matter most. Keep Gmail's reported spam rate low and steady.
- Use Postmaster Tools, DMARC reports, seed inboxes and bounce messages to see what providers see.
- Diagnose drops in order: authentication, bounces, volume, content, reputation.
- Recover a damaged domain by pausing, fixing the cause, resting it, and restarting low.
- Make opting out easy. It protects your reputation more than any trick.
Respuestas rápidas sobre la masterclass
¿Cuántos correos en frío debo enviar al día?
Empieza con 5-10 por bandeja al día en cuentas nuevas. Tras 2-3 semanas de calentamiento, sube a 35-50 por bandeja. Si necesitas enviar 500+ correos al día, usa 10-15 bandejas calentadas y rota los envíos entre todas. Nunca fuerces una sola bandeja más allá de 50 correos en frío al día.
¿Cuál es una buena tasa de respuesta en correo en frío?
No hay una cifra única y honesta. La tasa de respuesta depende de tu mercado, tu lista, tu oferta y de cómo se cuentan las respuestas, así que las cifras de campañas ajenas rara vez sirven. Compara cada campaña con tus propios resultados anteriores para el mismo tipo de público y valora las respuestas interesadas, no el total. El capítulo 12 explica cómo.
¿Qué longitud debe tener un correo en frío?
Mantén los correos en frío entre 50-125 palabras. Algunos estudios sugieren hasta 150 palabras para ciertas audiencias B2B. Los correos más cortos rinden mejor de forma constante. Si necesitas más de 125 palabras para exponer tu idea, tu mensaje no está lo bastante enfocado.
¿Debo usar HTML o texto plano en los correos en frío?
Texto plano. Siempre. Los correos en HTML con logos, imágenes y diseños con formato activan los filtros de spam y parecen correos de marketing. Un correo en texto plano de una persona a otra es lo que debería parecer un correo en frío. La única excepción es tu firma, que puede incluir un formato básico.
¿Cuántos seguimientos debo enviar?
Normalmente unos pocos, separados por varios días, y cada uno con algo nuevo: otro enfoque, un recurso útil o una pregunta sencilla. Para en cuanto alguien responda y termina con un cierre breve y educado. El capítulo 8 trata el ritmo y qué decir en cada paso.