Early AccessWe're giving lifetime free access to our first users - no credit card, no catch.Claim your spot →
Free Resource

The complete guide to email deliverability.

Deliverability affects every email you send. This guide covers DNS authentication, sender reputation, warm-up strategy, and everything in between.

99.2%
avg inbox placement
< 0.1%
target spam rate
30 days
typical warmup
3
auth protocols

What is email deliverability?

Email deliverability refers to the ability of an email to reach a recipient's inbox rather than their spam folder, promotions tab, or not at all. It is distinct from delivery rate - which measures whether the receiving server accepted the message - deliverability measures where that message lands.

You can have a 99% delivery rate and a 40% inbox placement rate. That gap is the deliverability problem. It silently destroys campaign performance, onboarding sequences, and transactional reliability.

Inbox providers evaluate hundreds of signals when deciding where to place your email. Sender reputation, authentication compliance, content quality, and list hygiene are the four primary dimensions.

Delivery vs. inbox placement

Delivery rate99%
Server accepted the message
Inbox placement40–95%
Landed in inbox (not spam)
The gap is the problem. 100% delivered ≠ 100% inbox.
Delivery rateDid the server accept the email?
Inbox placement rateDid it land in inbox vs. spam?
Sender reputationHow does the provider score your domain?
Spam rateHow often do recipients mark you as spam?

The fundamentals

The 4 pillars of deliverability

01

Authentication

SPF, DKIM, and DMARC prove to receiving mail servers that your email is legitimate. Without all three configured correctly, you are invisible to filtering systems - or worse, treated as a spoofing attempt.

SPFDKIMDMARC
02

Sender Reputation

Inbox providers maintain a reputation score for every sending IP and domain based on engagement - opens, clicks, replies, spam complaints, and unsubscribes. Low engagement pushes future sends toward spam.

IP reputationDomain reputationEngagement signals
03

Content Quality

Spam filters analyze body, subject line, HTML structure, image-to-text ratio, and links. Flagged words, broken HTML, URL shorteners, and misleading sender names all raise your spam score.

Subject lineHTML qualityLink analysis
04

List Hygiene

Sending to stale, purchased, or unengaged contacts dramatically increases bounce and spam complaint rates. A healthy list has regular re-engagement pruning, functioning unsubscribes, and double opt-in.

Bounce rateSpam complaintsOpt-in quality

Authentication deep dive

SPF, DKIM, and DMARC explained

SPFPASS
DKIMPASS
DMARCPASS
SPFSender Policy Framework

What it is

SPF lets you publish a list of IP addresses and servers authorized to send email on behalf of your domain.

How it works

The receiving server checks the DNS record of the sending domain and verifies the sending IP is listed. If not, the message fails SPF.

Example SPF DNS TXT record

v=spf1 include:sendgrid.net include:mailgun.org ~all
DKIMDomainKeys Identified Mail

What it is

DKIM adds a cryptographic signature to the headers of outgoing messages, verified by the recipient using a public key in your DNS.

How it works

Your sending server signs each message with a private key. The receiving server fetches the public key from your DNS and verifies the signature hasn't been altered in transit.

Example DKIM DNS TXT record (selector: mail)

mail._domainkey IN TXT "v=DKIM1; k=rsa; p=MIIBIjANBgkqhkiG9w0BAQ..."
DMARCDomain-based Message Authentication, Reporting, and Conformance

What it is

DMARC ties SPF and DKIM together and tells receiving servers what to do when authentication fails - nothing, quarantine, or reject.

How it works

Publish a DMARC record with your policy. Receiving servers send you daily XML reports showing pass/fail statistics, which domains are sending on your behalf, and how many messages were quarantined.

Example DMARC DNS TXT record (at _dmarc subdomain)

v=DMARC1; p=quarantine; rua=mailto:dmarc@yourdomain.com; pct=100

Warm-up strategy

Email warm-up: why it matters and how to do it right

When you start sending from a new domain or IP, inbox providers have no sending history to evaluate. Sending large volumes immediately mirrors the behavior of spammers who register new domains and blast millions of emails before being blocked.

Warm-up is the process of gradually increasing volume while maintaining high engagement. Each positive interaction - an open, a reply, a move from spam to inbox - signals to the provider that your mail is wanted.

Skipping warm-up on a new domain is the single most common reason cold outreach campaigns fail in the first week.

Typical 30-day ramp-up curve

Week 120–50 / day
Seed accounts only. Manual or AI-paced warm-up.
Week 250–200 / day
Gradual ramp. Monitor engagement closely.
Week 3200–500 / day
Expand to trusted lists if reputation holds.
Week 4+500–2,000+ / day
Full campaign volume. Maintain consistent send cadence.
Important: Never increase daily volume by more than 20–30% on any given day. Consistency matters more than speed.

What to avoid

10 spam triggers to avoid

Any one of these can push your email to spam regardless of everything else you do right.

ALL CAPS subject line

#01

Universally flagged by heuristic filters as aggressive and spammy.

No unsubscribe link

#02

Required by CAN-SPAM and GDPR. Absence is a hard spam signal for filters and ISPs.

URL shorteners (bit.ly, tinyurl)

#03

Shared shortener domains carry reputation pollution and obscure the destination URL.

Spam-trigger words in subject

#04

"Free", "Win", "Click now", "Guaranteed", "No risk" - common in filter training sets.

Image-heavy, text-light emails

#05

Low text-to-image ratio is a classic phishing pattern. Aim for at least 40% text.

Misleading "From" name

#06

Sender name that does not match the sending domain causes DMARC misalignment and user spam reports.

Broken or missing HTML tags

#07

Malformed HTML signals the email was not carefully crafted - a common spammer shortcut.

Multiple exclamation marks!!!

#08

Heuristic filters weight punctuation abuse heavily, especially in subject lines.

Attachments on cold email

#09

Unsolicited attachments drastically increase spam score. Links to hosted files are safer.

Sending to a purchased list

#10

Purchased lists contain spam traps, invalid addresses, and people who never opted in.

Reference

Deliverability glossary

Bounce rate
The percentage of sent emails not successfully delivered. Hard bounces are permanent failures (bad address). Soft bounces are temporary (full mailbox).
Spam rate
The percentage of delivered emails marked as spam. Google's bulk sender guidelines require keeping this below 0.1%, with a 0.3% hard threshold.
Inbox placement rate
The percentage of successfully delivered emails that landed in the inbox (not spam or promotions). Delivery rate and inbox placement rate are different metrics.
Sender reputation
A composite score assigned by each inbox provider based on engagement history, complaint rates, blacklist status, authentication compliance, and sending consistency.
DMARC
Domain-based Message Authentication, Reporting, and Conformance. A policy layer on top of SPF and DKIM that tells servers how to handle authentication failures.
DKIM
DomainKeys Identified Mail. Adds a cryptographic signature to outgoing email headers that the receiving server can verify using a public key in your DNS.
SPF
Sender Policy Framework. A DNS record listing IP addresses authorized to send email on behalf of your domain. Prevents unauthorized servers from forging your sender address.
MX record
Mail Exchange record. A DNS record specifying which mail server receives email for your domain. Does not directly affect outbound deliverability.
Blacklist
A database of IP addresses or domains known to send spam. Major blacklists include Spamhaus, Barracuda, and SORBS. Being listed causes widespread inbox blocking.
Warm-up
Gradually increasing email send volume from a new domain or IP over 2–6 weeks to build a positive reputation history before sending at full volume.

Check your deliverability - free.

Connect a mailbox and get an instant report on your authentication setup, sender reputation, and inbox placement rate.

Get started free

Send like the inbox is yours.

Start for free - no credit card required.

Get started freeBook a demo