Last updated: October 2026.
Quick answer: Improving email deliverability comes down to five fixable problems: weak authentication, poor sender reputation, dirty contact lists, erratic sending behavior, and content that trips spam filters. Diagnose which one you have before you change anything. Mailivery breaks down how to spot and fix each one.
If your open rates dropped last month, or a campaign that used to land in the inbox now lands in spam, you're probably searching for a checklist of fixes. Google has dozens of those. Throwing every best practice at a deliverability issue won't solve it, though. You end up rewriting subject lines when the real issue was a DMARC policy, or warming up an inbox when the real issue was a list of dead addresses.
Five specific things break deliverability, and the fix depends on which one you have. This guide helps you diagnose the problem first, then matches each symptom to the right fix: authentication, sender reputation (where email warmup does most of the work), list hygiene, sending behavior and content. You'll also get 2026 benchmarks and the signs it's time to bring in a deliverability platform. If you want a general checklist instead, see our 25 email deliverability best practices.
Email delivery means the recipient's mail server accepted your message. Email deliverability means the message landed in the inbox instead of the spam or promotions folder. Most email tools report a 99% delivery rate, which sounds great until you realize half of those emails could be sitting in spam.
The fixes differ too. A bounced email is a delivery problem. An email that arrived in spam is a deliverability problem. Mixing up the two is how people end up buying better lists when the real fix was a missing DMARC record.
A healthy 2026 deliverability profile has a bounce rate under 2%, a spam complaint rate under 0.1% (Gmail starts enforcing at 0.3%), inbox placement around 80% or higher on a placement test, and engagement trending up. If any of those numbers is off, start diagnosing.
Most senders have one or two of the five problems, and fixing the wrong one wastes weeks. If you're not sure how to test email deliverability, match your symptoms below to narrow it down, then confirm with free tools like MXToolbox and Google Postmaster Tools.
Symptom: bounces with errors like "domain not authenticated," or messages rejected outright. You have an authentication problem. Your SPF, DKIM or DMARC records are missing or misconfigured. Check them with MXToolbox or the Google Admin Console, and if any of the three fail, start there.
Symptom: open rates were decent a few months ago and have dropped month after month since. You have a sender reputation problem. Low engagement, spam complaints or sudden volume changes have added up to enough negative signals that mailbox providers now filter you harder. Check Google Postmaster Tools for your domain reputation and spam complaint rate. A complaint rate climbing toward 0.3% confirms it.
Symptom: bounce rate above 2%, with hard bounces making up most of it. You have a list quality problem. Your list contains invalid addresses, role accounts (info@, support@), spam traps or contacts who haven't engaged in months. A single spam trap can damage your whole domain, and high bounces drag reputation down fast.
Symptom: emails land in spam right after a campaign launch, then recover. You have a sending behavior problem. Volume spikes, a new domain with no warm-up, or one email blasted to thousands of recipients at once all look suspicious to mailbox providers.
Symptom: some emails go to spam while others from the same sender land fine. You have a content problem: spam trigger words, too many links, image-heavy layouts or copy that resembles known spam. Mail-Tester scores individual emails for spam risk.
Symptom: every metric dropped at once. Check whether you're on a blacklist. MXToolbox checks the major lists for free, and a Spamhaus listing goes to the top of your fix list. If your domain or IP sits on more than two major blacklists, replacing it usually costs less than recovering it.
For more on the spam-folder side, why your emails are going to spam covers that diagnosis in more detail.
Authentication proves you are who you claim to be, and mailbox providers treat unauthenticated senders as suspicious by default. In February 2024, Google and Yahoo made authentication a requirement for bulk senders. Microsoft followed on May 5, 2025, for domains sending more than 5,000 emails a day to Outlook.com addresses. If your authentication is incomplete, that alone can explain poor deliverability.
The three records you need:
SPF (Sender Policy Framework) lists the mail servers authorized to send on your behalf. When a recipient's server gets your email, it checks your SPF record to confirm the sending server is on your approved list.
DKIM (DomainKeys Identified Mail) adds a cryptographic signature to your emails that proves nobody altered them in transit. The receiving server verifies the signature against a public key published in your DNS.
DMARC (Domain-based Message Authentication, Reporting, and Conformance) tells receiving servers what to do when SPF or DKIM checks fail. A policy of p=none is the minimum for bulk sender compliance. A policy of p=quarantine or p=reject is becoming the standard for trusted senders in 2026.
If you haven't set these up, the full SPF, DKIM, and DMARC setup guide walks through the records step by step for Google Workspace, Microsoft 365 and most major providers. Don't skip DMARC. Mailivery won't start warm-up on a mailbox that fails its DNS check, because warming an unauthenticated mailbox wastes the effort.
To verify your records, run your domain through MXToolbox's deliverability checker. Each record should pass, and if any fail, fix that before anything else. Mailivery's header analyzer also shows whether a real campaign email passed SPF, DKIM and DMARC alignment.
Sender reputation is the score mailbox providers assign to your domain (and sometimes your IP) based on how recipients interact with your email over time. Opens, replies and moves out of spam build it. Deletes, spam complaints and silence erode it, and one bad campaign can undo weeks of progress.
An email warmup tool is the fastest way to build reputation on purpose. Warm-up generates the engagement signals mailbox providers score you on. Without it, building those signals from real sending alone takes months.
Mailivery's email warmup runs through a peer-to-peer network of over 100,000 active mailboxes, with AI-generated conversations and the volume and reply rate under your control. Mailivery supports both B2B and B2C warm-up on every plan, which matters if you're warming inboxes for ecommerce, newsletters or transactional mail alongside outreach. Our recommended email warmup settings cover the exact numbers: warm-up matched 1:1 to your cold volume, a 35 to 40% reply rate, and 1 month of warm-up before your first campaign (2 weeks minimum).
Two rules apply whether or not you use warm-up:
For marketing email senders: keep your spam complaint rate under 0.1%. Gmail enforces a 0.3% threshold, and senders who cross it see aggressive filtering.
For cold email senders: your reputation depends on positive replies and conversation depth. Cold recipients engage less by default, so warm-up matters more for outbound senders. Pair it with verified lists and personalized first lines that earn replies.
Reputation is tracked per domain. Send cold campaigns from a separate domain so one bad campaign can't burn your primary domain, and keep each outreach domain to 3 mailboxes (5 at most).
A dirty list undermines every other fix. Invalid addresses, spam traps and disengaged recipients push up bounce rates, drag down engagement and hurt your reputation, even with perfect authentication and warm-up.
Do these in order:
Verify every address before you send. Run new contacts through email verification before they go on your list. Verification catches invalid syntax, dead domains and known spam traps, and it's the most effective way to keep bounces under 2%. Mailivery includes email verification credits on every plan, so you don't need a separate tool.
Remove disengaged subscribers. If a contact hasn't opened or clicked anything in 90 to 180 days, suppress them. Mailbox providers track engagement at the recipient level, and ten thousand people who never open your email tell Gmail you're unwanted, even if none of them complain.
Remove hard bounces right away. A hard bounce means the address doesn't exist. If you keep sending to it, mailbox providers conclude you don't manage your list. Most email tools suppress hard bounces automatically, but check that yours suppresses them instead of retrying.
Mailbox providers can tell whether you send like a person or a script, which makes sending behavior one of the hardest signals to fake. Gradual, consistent sending earns trust, and sudden spikes or machine-timed bursts trigger filters.
For marketing email, consistency matters most. Send on a regular schedule, stagger large sends instead of hitting your whole list in the same minute, use send-time optimization if your tool has it, and ramp volume up in steps when you scale a new campaign.
For cold email, the rules are stricter:
Content matters least of the five. Modern spam filters weigh engagement more than keyword lists, so an email with a few "spam words" that gets opened and replied to will outperform a clean email that gets ignored. Content still matters at the margins.
Check these:
Image-to-text ratio. Emails that are mostly images (or one big image with no text) trigger filters. Aim for at least 60% text by content area.
Link count. In cold email, use one link at most and leave it untracked. Marketing emails can carry more links, but skip URL shorteners like bit.ly, which spam filters distrust.
Open and click tracking. Our deliverability team sees tracking pixels and tracked links as one of the main reasons cold email lands in spam. If replies matter more to you than opens (they should for cold email), turn tracking off. If you do track clicks, use a custom tracking domain on your own subdomain instead of your tool's shared default.
Template repetition. Mailbox providers fingerprint a template sent at scale in about a month. Rotate your copy with spintax and personalization so no two sends look identical.
Run individual emails through Mail-Tester to get a content score. A 9 or 10 out of 10 means the content side is fine. Below 7, fix what the report flags before you send.
If you're wondering how to check email deliverability over time, track these benchmarks and the thresholds mailbox providers enforce.
Check Google Postmaster Tools weekly if you send meaningful volume to Gmail. It shows your domain reputation, IP reputation, spam rate and authentication status, all from Google's own filtering data. Microsoft's SNDS (Smart Network Data Services) gives similar visibility for Outlook and Hotmail. Both are free. Judge your reputation by Gmail results first, since Outlook placement is unpredictable for most senders right now.
If you've worked through all five fixes and your numbers still lag, you likely need infrastructure that manages deliverability as an ongoing process.
Signs it's time for a deliverability platform:
Mailivery bundles email warmup, blacklist monitoring across 70+ lists, email verification and inbox placement testing on every plan. Pricing is based on warm-up volume, not mailboxes, so you connect as many mailboxes as you want for a flat monthly rate. Starters runs $29/month with a free 7-day trial, Professional is $79/month, and Business is $199/month for higher-volume teams and agencies. See pricing details.
Aim for inbox placement of 80% or higher on a placement test. Mailivery sees about 80% as normal for a healthy Google Workspace sender, and placement below 50% points to a reputation problem rather than a DNS problem. Don't confuse this with delivery rate, which often reads 98% or higher even when much of that mail lands in spam.
Run your domain through three free tools. MXToolbox verifies SPF, DKIM and DMARC and confirms you're not on a blacklist. Google Postmaster Tools shows your Gmail reputation and spam complaint rate. Mail-Tester scores individual emails for content issues. For a deeper view of where your emails land across providers, an inbox placement test sends to seed mailboxes and reports the results. Mailivery includes inbox placement testing on every plan.
The fastest single fix is usually authentication. If your SPF, DKIM or DMARC records are misconfigured or missing, fixing them can produce a noticeable improvement within 24 to 48 hours of DNS propagation. Removing disengaged contacts and hard bounces from your list is the next fastest. Reputation fixes (warm-up, sending behavior changes) take 2 to 6 weeks to show measurable impact.
Yes. Email warmup builds the positive engagement signals (opens, replies, conversations) that mailbox providers use to score sender reputation. Without warm-up, building those signals takes months, and a single bad campaign can erase the progress. Mailivery recommends 1 month of warm-up before a new mailbox's first campaign, with 2 weeks as the minimum, and running warm-up alongside your campaigns after that.
Authentication fixes resolve in 24 to 48 hours. List hygiene improvements show up within one or two campaign cycles. Reputation recovery takes longer: plan on 2 to 3 weeks for minor issues, 4 to 6 weeks for moderate damage, and 6 to 12 weeks for a domain with heavy damage. The timeline depends on how much damage you start with and how steady your sending stays during recovery.
Email delivery is whether the recipient's mail server accepted your message. Email deliverability is whether the message landed in the inbox instead of the spam or promotions folder. A 99% delivery rate tells you little if half of those emails ended up in spam. Most ESPs report delivery as their headline number, so senders often think their deliverability is fine when it isn't.
Yes, in most cases. Authentication, list hygiene, sending behavior and content fixes all happen at the domain or sender level, not the tool level. The exception is a tool with a poor shared IP reputation or limited deliverability features. For high-volume or cold outreach senders, a tool with built-in warm-up and blacklist monitoring usually outperforms a stack of separate single-purpose tools.
Check Google Postmaster Tools weekly. Run an inbox placement test before major campaigns. Audit authentication records monthly to catch DNS drift. Review bounce rate, spam complaint rate and engagement campaign by campaign. Most deliverability problems build for weeks before they show up in open rates, so check before campaigns, not after.
Mailivery bundles warmup, blacklist monitoring, verification and inbox placement testing on every plan, so you can run each check in this guide from one dashboard. Start a 7-day free trial on the Starters plan, or see how Mailivery warm-up works.
The easiest way to get responses to your cold emails.
