The complete guide to email deliverability.
Deliverability affects every email you send - from password resets to cold outreach. This guide covers everything from DNS authentication to warm-up strategy.
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 simply 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 - the emails that were accepted but filtered to spam - is the deliverability problem. It silently destroys campaign performance, onboarding sequences, and transactional reliability.
Inbox providers like Google, Microsoft, and Yahoo 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 they score. Weak performance on any one dimension can push your mail into spam even if the others are perfect.
Key terms
The fundamentals
The 4 pillars of deliverability
Authentication
SPF, DKIM, and DMARC are the three protocols that prove to receiving mail servers that your email is legitimate and was sent by who it claims. Without all three configured correctly, you are invisible to filtering systems - or worse, treated as a spoofing attempt.
Sender Reputation
Inbox providers maintain a reputation score for every sending IP and domain they see. This score is based on engagement signals - opens, clicks, replies, spam complaints, and unsubscribes. A consistently low engagement rate signals low-quality mail, pushing future sends toward the spam folder.
Content Quality
Spam filters analyze the body, subject line, HTML structure, image-to-text ratio, and links inside your email. Flagged words, broken HTML, URL shorteners, and misleading sender names are all content-level signals that raise your spam score.
List Hygiene
Sending to stale, purchased, or unengaged contacts dramatically increases your bounce and spam complaint rates. A healthy list has regular re-engagement pruning, functioning unsubscribe flows, and double opt-in at collection time.
Authentication deep dive
SPF, DKIM, and DMARC explained
What it is: SPF lets you publish a list of IP addresses and servers that are 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 that the sending IP is listed. If not, the message fails SPF.
Example SPF DNS TXT record
v=spf1 include:sendgrid.net include:mailgun.org ~allWhat it is: DKIM adds a cryptographic signature to the headers of outgoing messages. This signature is verified by the recipient using a public key published in your DNS.
How it works: Your sending server signs each message with a private key. The receiving server fetches the corresponding public key from your domain's 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..."What it is: DMARC ties SPF and DKIM together and tells receiving servers what to do when authentication fails - nothing, quarantine, or reject. It also enables aggregate reports sent to your inbox.
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 or rejected.
Example DMARC DNS TXT record (at _dmarc subdomain)
v=DMARC1; p=quarantine; rua=mailto:dmarc@yourdomain.com; pct=100Warm-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 is a major red flag - it 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. Over 30 days, this builds a reputation history that makes your domain trusted at scale.
Skipping warm-up on a new domain is the single most common reason cold outreach campaigns fail in the first week. Even if your content is perfect and your list is clean, the absence of domain history means immediate heavy filtering.
Typical 30-day ramp-up curve
What to avoid
10 spam triggers to avoid
ALL CAPS subject line
Universally flagged by heuristic filters as aggressive and spammy.
No unsubscribe link
Required by CAN-SPAM and GDPR. Absence is a hard spam signal for filters and ISPs.
URL shorteners (bit.ly, tinyurl)
Shared shortener domains carry reputation pollution from other senders and obscure the destination URL.
Spam-trigger words in subject
"Free", "Win", "Click now", "Guaranteed", "No risk" - these phrases are common in filter training sets.
Image-heavy, text-light emails
A low text-to-image ratio is a classic phishing and spam template pattern. Aim for at least 40% text.
Misleading "From" name
Sender name that does not match the actual sending domain causes DMARC misalignment and user-level spam reports.
Broken or missing HTML tags
Malformed HTML is a signal that the email was not carefully crafted, a common spammer shortcut.
Multiple exclamation marks!!!
Heuristic filters weight punctuation abuse as a strong spam indicator, especially in subject lines.
Attachments on cold email
Unsolicited attachments drastically increase spam score. Links to hosted files are safer.
Sending to a purchased list
Purchased lists contain spam traps, invalid addresses, and people who never opted in. A single send can cause a blacklist hit.
Reference
Deliverability glossary
- Bounce rate
- The percentage of sent emails that were not successfully delivered. Hard bounces are permanent failures (bad address). Soft bounces are temporary (full mailbox). High bounce rates damage sender reputation.
- Spam rate
- The percentage of delivered emails that recipients marked as spam. Google's bulk sender guidelines require keeping this below 0.1%, with a hard threshold of 0.3% before deliverability penalties kick in.
- Inbox placement rate
- The percentage of successfully delivered emails that landed in the inbox (not spam folder or promotions tab). Delivery rate and inbox placement rate are different metrics - 100% delivered does not mean 100% inbox.
- Sender reputation
- A composite score assigned to a sending IP and domain 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 receiving servers how to handle authentication failures and sends reporting data back to the sender.
- 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, confirming the message was not tampered with in transit.
- SPF
- Sender Policy Framework. A DNS record that lists the IP addresses and services authorized to send email on behalf of your domain. Prevents unauthorized servers from forging your sending address.
- MX record
- Mail Exchange record. A DNS record that specifies which mail server is responsible for receiving email for your domain. Required for inbound email. 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 until you request removal.
- Warm-up
- The process of gradually increasing email send volume from a new domain or IP over 2–6 weeks. Allows inbox providers to observe consistent, low-complaint sending before the domain sends 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