Free Email
Deliverability Course

Email deliverability is the ability of an email to reach its intended recipient's inbox — not spam, not promotions, not junk. It is measured as inbox placement rate (IPR), which is distinct from the more commonly reported delivery rate.

How ISPs decide where your email lands

Internet Service Providers (Gmail, Outlook, Yahoo, Apple Mail) use a combination of signals to decide whether your email belongs in the inbox, spam, or promotions tab — or should be blocked entirely:

• Sender reputation — the trust score assigned to your sending IP address and domain based on past behaviour.
• Authentication — whether your emails are signed with DKIM, pass SPF checks, and are protected by DMARC.
• Engagement signals — how often your recipients open, click, reply to, and move your emails to the inbox vs. marking them as spam.
• Content signals — spam trigger words, image-to-text ratio, broken HTML, misleading subject lines.
• List quality — your bounce rate, spam complaint rate, and whether you're hitting spam traps.

Email authentication is a set of DNS-based standards that prove to receiving mail servers that your emails are genuinely from your domain and haven't been tampered with. Without authentication, your emails are treated as unverified and untrustworthy.

SPF — Sender Policy Framework

SPF specifies which IP addresses and mail servers are authorised to send email on behalf of your domain. It is a TXT record in your domain's DNS.

DKIM — DomainKeys Identified Mail

DKIM adds a cryptographic signature to every outbound email. The receiving server uses your public key (stored in DNS) to verify the signature, confirming the email genuinely came from your domain and wasn't modified in transit.

DMARC — Domain-based Message Authentication

DMARC sits on top of SPF and DKIM. It tells receiving servers what to do when an email fails authentication — and sends you reports about who is sending email using your domain.

Every new dedicated IP address starts with zero reputation. ISPs have never seen it before — so they treat it with maximum suspicion. IP warming is the process of gradually increasing your sending volume to give ISPs time to observe your behaviour and build trust in your IP.

Skip the warm-up and send full volume immediately, and your emails will go directly to spam — or be blocked entirely. Recovery from a burned IP can take 30–90 days.

The Warm-up Principle

• Start with your most engaged subscribers — those who opened or clicked in the last 30 days.
• Low volume + high engagement signals to ISPs that you send wanted, legitimate email.
• ISPs track the ratio of complaints, bounces, and engagement on every IP. High engagement = positive reputation building.
• Double or triple your daily volume each week, never jumping more than 3× in a single day.
• Monitor bounce rates and spam complaint rates daily throughout the warm-up period.

Sample 6-Week Warm-Up Schedule

Use the Email Warm-Up Calculator to generate a custom schedule based on your specific list size and target volume.

A dirty list is the single fastest way to destroy sender reputation. Hard bounces, spam traps, role-based addresses, and long-term unengaged subscribers all signal to ISPs that you are not managing your list responsibly.

What to remove and why

Re-engagement before suppression

Before suppressing unengaged subscribers (those who haven't opened or clicked in 12+ months), run a 3-email re-engagement sequence:

• Email 1 (Week 1): "We miss you" — remind them who you are and what's new. No offer yet.
• Email 2 (Week 2): Offer a meaningful incentive — discount, free content, exclusive access.
• Email 3 (Week 3): "Last chance — should we keep in touch?" Re-permission email with a single CTA to stay on the list.

Anyone who doesn't engage with all three emails should be suppressed. Keep them in your database but remove them from all future sends. Your list will be smaller but dramatically healthier.

An email blacklist (also called a Real-time Blackhole List or RBL) is a database of IP addresses and domains known to send spam. When a receiving mail server receives an email from a blacklisted IP or domain, it either rejects the email outright or sends it to spam.

The most impactful blacklists to monitor

How to get delisted

Step 1: Identify why you were listed. Before requesting removal, understand the cause. Common reasons: spam complaints over threshold, spam trap hit, sending to purchased lists, authentication failure, sudden volume spike.

Step 2: Fix the root cause. Clean your list. Remove the spam trap sources. Fix authentication. Requesting removal without fixing the cause will result in re-listing within days.

Step 3: Submit a removal request. Each blacklist has its own delisting process. Spamhaus requires completing their Self-Service Removal (SSR) portal. Barracuda requires submitting through barracudacentral.org. Most removals take 24–72 hours once approved.

Even with perfect authentication and a clean list, poorly constructed email content can cause spam filters to flag your messages. Modern filters are sophisticated — they use machine learning on recipient engagement data combined with content analysis.

Subject line best practices

• Keep subject lines 40–60 characters — long enough to be descriptive, short enough not to truncate on mobile.
• Avoid ALL CAPS — capitalising entire words or subject lines is a major spam trigger across all filters.
• No excessive punctuation — multiple exclamation marks (!!!) or question marks (???) trigger filters.
• Avoid classic trigger phrases — "FREE", "Act now", "Limited time", "You've been selected", "No cost", "Guaranteed".
• Personalise where appropriate — subject lines with the recipient's first name have lower spam complaint rates.
• Match subject to content — misleading subject lines not only trigger spam filters but also increase complaint rates.

HTML email structure

Deliverability is not a one-time setup — it requires ongoing monitoring. Sender reputation fluctuates based on campaign performance, list changes, ISP algorithm updates, and sending behaviour. Build a regular monitoring routine to catch problems early.

Your deliverability dashboard — metrics to track

Recommended monitoring schedule

Essential tools