# How to Improve Email Deliverability in 2026: The Complete Guide URL: https://warmerly.com/blog/how-to-improve-email-deliverability Published: 2026-06-15 Reading time: 14 minutes Tags: Deliverability, Cold email, Guide > A step-by-step guide to improving email deliverability in 2026. Covers domain setup, authentication, warmup, sender reputation, list hygiene, content, and inbox placement testing. Deliverability is the gap between sending an email and having someone read it. You can write the perfect sequence, build an accurate list, and pick the right timing — and still get nothing back if your messages are landing in spam or being silently blocked before they arrive. This guide covers every layer of deliverability in the order you should tackle it. Start at the top and work down. Each section links to deeper guides where relevant. The goal is a mailbox that reliably reaches the inbox, stays there, and keeps your sender reputation healthy as you scale. > **Who this is for** — Cold email senders, sales teams, agencies, and founders setting up or improving an outbound email system. Whether you are starting from scratch or fixing a broken setup, this guide covers the full picture. ## 1. Use a dedicated sending domain The first rule of cold outreach: never send from your main business domain. If something goes wrong — a spam complaint spike, a blacklisting, a Gmail policy violation — you want that problem contained to a sending domain, not attached to the domain you use for everything else. Register a variation of your main domain specifically for cold outreach. Common formats: add a prefix (mail.yourdomain.com), use a hyphen (your-domain.com), or register a different TLD (.io, .co). The sending domain should still look professional and recognisable, but be separate from your primary one. - Buy the domain from a reputable registrar (Namecheap, Google Domains, Cloudflare). - Set up MX records pointing to your mail provider before anything else. - Age the domain for at least two to three weeks before sending. New domains are flagged by default. - Avoid expired domains. Their history is opaque and often carries residual damage from previous senders. ## 2. Configure authentication: SPF, DKIM, DMARC, and MTA-STS Authentication is the baseline. Without it, even a perfectly warmed mailbox will get spam-foldered by Gmail and Outlook. Authentication records are DNS entries that prove you are who you say you are. There are four you need to configure before any warmup or campaign starts. ### SPF SPF (Sender Policy Framework) is a TXT record at your domain apex that lists which mail servers are authorised to send on your behalf. One record, under 10 DNS lookups, ending with ~all (soft fail) while you verify, then -all (hard fail) once you are confident. Multiple SPF records on the same domain is an automatic failure. ### DKIM DKIM (DomainKeys Identified Mail) cryptographically signs each outbound message. Your mail provider generates a public/private key pair. You publish the public key in DNS and the provider signs messages with the private key. Use 2048-bit keys — 1024-bit is considered weak in 2026. ### DMARC DMARC tells receivers what to do when SPF or DKIM fails. Start at p=none (monitor only) for the first 30 days and set up aggregate reporting to a mailbox you actually read. Once reports show no legitimate sources failing, move to p=quarantine, then p=reject. ### MTA-STS MTA-STS enforces TLS encryption on inbound mail. Gmail and Microsoft both treat it as a trust signal. It requires a policy file at a specific HTTPS URL on your domain plus a TXT record in DNS. Most sending domains skip this step, which is exactly why having it is a differentiator. After setting all four up, send a test message to a Gmail account you control. Open the original headers and check that SPF, DKIM, and DMARC all read PASS. Fix any failures before moving to warmup. ## 3. Warm up your mailboxes before sending Warmup is the single most skipped step in cold email setup, and it is the one that causes most early-stage deliverability failures. Gmail and Outlook have no reason to trust a new mailbox with zero history. Send 50 cold emails on day one and you trip every bulk-sender heuristic at once. Warmup works by generating realistic email activity on your mailbox before real sends start. A warmup tool connects your mailbox to a network of peer mailboxes, sends conversational messages, and has peers reply, star, and rescue messages from spam. Each positive interaction builds the sender reputation and engagement signals that inbox placement depends on. - New domain (under 30 days old): minimum 21 to 28 days of warmup before any cold sends. - Aged domain (clean history): 7 to 14 days is usually enough. - Previously burnt domain: may never fully recover — a new domain is often faster. - Keep warmup running at reduced volume while campaigns are active. Reputation decays without consistent sending. The strongest warmup signal is spam-folder rescue: a peer moving your message from the spam folder back to the inbox. This is the action Gmail weights most heavily in reputation scoring. Not all warmup tools perform this step on every peer message — it matters that yours does. > **The minimum viable warmup check** — Before your first cold send, your mailbox health score should be consistently above 80, inbox placement should be above 90% across the providers your prospects use, and your spam rescue rate should be trending down. If any of those three are off, give it more time. ## 4. Build and protect sender reputation Sender reputation is the score that providers assign to your domain and sending IP based on your sending history. It is not a single number with a dashboard — it is a collection of signals that each provider tracks independently. High reputation means inbox. Low reputation means spam or silent blocking. The signals that build reputation in 2026, roughly in order of weight: 1. Recipient engagement — opens, replies, stars, and spam rescues from real recipients. 2. Spam complaint rate — Google's sender guidelines recommend staying under 0.1%, and anything above 0.3% triggers active filtering. 3. Bounce rate — hard bounces above 2% signal poor list hygiene and damage domain reputation quickly. 4. Volume consistency — steady sending growth over time, not spikes. 5. Authentication pass rate — SPF, DKIM, and DMARC all passing on every message. Check your domain reputation in Google Postmaster Tools (free, takes five minutes to verify). The Domain Reputation tab will tell you if you are at High, Medium, Low, or Bad. Medium or below means active filtering. Low or Bad means almost everything you send will land in spam regardless of content. ## 5. Keep your list clean before every send A dirty list is one of the fastest ways to destroy sender reputation. Every hard bounce and every spam complaint is a negative signal that accumulates against your domain. A list of 500 verified, engaged contacts will almost always outperform a list of 5,000 unverified ones. - Run every list through an email verification tool before sending. Remove hard bounces, role addresses (info@, support@, admin@), and catch-all detections. - Monitor your bounce rate after every send. Stay under 2% hard bounces. Above that, stop and clean the list before the next campaign. - Remove anyone who has marked you as spam immediately. Most campaign tools handle this automatically, but verify the setting. - Never buy lists. They are full of role addresses, spam traps, and contacts who have never heard of you. - Re-verify lists older than 90 days. Email addresses go stale faster than most senders expect. ## 6. Write emails that reach the inbox Content is not the first problem to fix — authentication and reputation matter more — but it becomes the limiting factor once the other layers are sorted. Modern spam filters use machine learning models trained on billions of messages, not word lists. They look at structure and patterns, not individual words. What actually triggers spam classification in 2026: - High link-to-text ratio. One link per email is the safe ceiling for cold outreach. More than that and content filters push toward promotions or spam. - Redirect chains and URL shorteners. Use a direct link to your domain, not a link that bounces through a tracker and a shortener. - Image-heavy or HTML-heavy messages. Plain-text or near-plain-text emails consistently outperform designed HTML for cold outreach. - Mismatched sender name and From address. The display name should match or clearly relate to the sending address. - No plain-text version. Any HTML email should include a plain-text alternative in the MIME envelope. - Unsubscribe header missing on emails sent to lists. Gmail and Yahoo now require this for bulk senders. The practical rule: write cold emails that look like a real human wrote them from their personal inbox. Short, direct, one clear ask, one or no links, no graphics. ## 7. Ramp send volume gradually Volume spikes are one of the most reliable ways to trigger spam filtering. Providers look at rate of change, not just absolute volume. Jumping from 20 to 200 sends per day in a week looks like a compromised account, even if each individual message is perfectly crafted. A safe ramp for a warmed domain and mailbox: - Week 1 of cold sending: 10 to 20 emails per day per mailbox. - Week 2: 25 to 40 per day. - Week 3: 40 to 60 per day. - Week 4 onward: scale by 20% per week, not per day. - Keep warmup running at 30 to 40% of campaign volume throughout. If you need high-volume outreach, use multiple mailboxes across multiple sending domains rather than pushing one mailbox above its natural ceiling. Gmail's practical limit per mailbox for cold outreach is around 50 to 80 messages per day before risk rises meaningfully. ## 8. Monitor for problems before they compound Deliverability problems are much easier to fix early than after they have compounded across weeks of sending. Most senders discover a spam problem when their reply rate drops — by that point, their domain reputation may already be damaged. Set up these monitoring tools and check them weekly: - Google Postmaster Tools — domain reputation, spam rate, delivery errors, and feedback loop data. Free and the most reliable signal for Gmail deliverability. - Microsoft SNDS (Smart Network Data Services) — the equivalent for Outlook and Microsoft 365. - MXToolbox or similar — blacklist monitoring across the major RBLs. A single listing on Spamhaus can stop delivery to enterprise addresses. - Inbox placement seeds — maintained in Warmerly and run automatically with each warmup cycle. Check Gmail, Outlook, and Yahoo separately. - Bounce and complaint rate tracking in your campaign tool — anything above thresholds should pause sends automatically. Weekly checks catch 90% of problems before they become campaign-killing. The remaining 10% usually surface through a sudden drop in open rate, which is why comparing week-on-week open rates by recipient domain is worth doing after every significant send. ## 9. Common deliverability mistakes to avoid - Sending from your main business domain. One spam complaint away from damaging your company's email reputation permanently. - Skipping authentication and starting warmup. Warmup on a misconfigured domain is wasted effort. Authentication must pass before warmup starts. - Buying aged domains. The cheapest ones are cheap because their sender history is bad. Start fresh. - Stopping warmup when campaigns start. Reputation without maintenance decays over weeks. Run warmup continuously at reduced volume. - Scaling volume too fast. 20% per week maximum. Anything faster looks like a compromised account. - Never testing inbox placement. Open rates do not tell you where email landed. Seed testing does. - Assuming Gmail and Outlook behave the same. They use different signals with different weights. Always test both. - Treating deliverability as a one-time setup. It is ongoing maintenance, not a checkbox. --- Source: https://warmerly.com/blog/how-to-improve-email-deliverability Full content index: https://warmerly.com/llms-full.txt Site index: https://warmerly.com/llms.txt