How to Fix Low Email Deliverability: 11 Reasons Your Emails Land in Spam

Low email deliverability is rarely caused by one dramatic mistake. It is almost always the result of three or four small issues stacking up: a DKIM record that stopped signing, a list that has not been cleaned in two years, and a subject line that trips filters. The good news is that every one of those issues is measurable, and most can be fixed in an afternoon.

This guide is built as a diagnostic. Work through the eleven causes below in order, apply the fix, then use the free verification method attached to each one to confirm the problem is actually gone. No guessing, no “try sending fewer emails and hope”.

First, Delivery Is Not Deliverability

These two words get mixed up constantly, and confusing them is why so many teams think everything is fine while revenue quietly drops.

Metric	What it measures	What it hides
Delivery rate	The receiving server accepted the message (it did not bounce)	The message may have gone straight to Spam or Promotions
Deliverability (inbox placement)	The message reached the primary inbox	Nothing. This is the number that matters

You can have a 99.4% delivery rate and a 61% inbox placement rate at the same time. Your ESP dashboard will look healthy. Your revenue will not.

What Counts as Low Email Deliverability?

Inbox placement rate	Verdict	Action
98% and above	Excellent	Monitor monthly
85% to 97%	Acceptable	Tune list hygiene and segmentation
70% to 84%	Low	Run the full diagnostic below
Below 70%	Critical	Pause campaigns, fix authentication, rebuild sending reputation

The 10-Minute Triage Before You Read Any Further

Do this before touching anything. It tells you whether your problem is technical, reputational or behavioural. There’s a good explainer over at twilio.com.

1. Send a campaign to a seed address on Gmail. Open it, click the three dots, choose Show original. You want to see PASS next to SPF, DKIM and DMARC.
2. Send the same email to mail-tester.com and note the score out of 10. Below 8 means content or configuration issues.
3. Open Google Postmaster Tools and check the Spam Rate graph for the last 30 days. Anything above 0.3% is a red alert.
4. Run your sending domain and sending IP through MXToolbox Blacklist Check.
5. Pull your bounce rate and unsubscribe rate for the last five campaigns.

Now match what you found to the causes below.

Cause 1: Your SPF Record Is Missing, Wrong or Overloaded

Symptom: spf=fail, spf=softfail or spf=permerror in the message headers.

SPF tells receiving servers which servers are allowed to send mail using your domain. Two things break it constantly. First, teams add a new sending tool (a CRM, a helpdesk, an invoicing platform) and forget to include it. Second, the record exceeds the hard limit of 10 DNS lookups, which produces a permerror and invalidates the whole record even if everything else is correct.

The Fix

• Publish a single SPF TXT record per domain. Two SPF records equals zero valid SPF records.
• List every legitimate sender, then end with -all (hard fail) rather than ~all once you are confident the list is complete.
• If you are over 10 lookups, replace bulky include: statements with the specific IP ranges you actually use, or use an SPF flattening service.

Verify it: MXToolbox SPF Record Lookup shows the lookup count and flags syntax errors. Re-send a seed email and confirm spf=pass in Show original.

Cause 2: DKIM Is Not Signing, or the Key Is Too Weak

Symptom: dkim=none, dkim=fail, or DKIM passing for a domain that is not yours (your ESP’s domain instead of your own).

DKIM adds a cryptographic signature that proves the message was not altered in transit and genuinely came from your domain. A surprising number of accounts have DKIM “enabled” in the ESP interface while the DNS record was never actually published, or was published on the wrong hostname.

The Fix

• Publish the CNAME or TXT selector record exactly as your ESP provides it, including the selector prefix.
• Use a 2048-bit key. 1024-bit keys still pass but are increasingly treated as weak.
• Make sure the signing domain (d= in the header) matches your From domain. If d= shows your ESP, you are using their shared reputation, not building your own.
• Rotate keys at least once a year.

Verify it: In Show original, confirm dkim=pass and that header.d= is your own domain. MXToolbox DKIM Lookup will resolve your selector directly.

Cause 3: No DMARC Record, or a Policy Stuck at p=none Forever

Symptom: dmarc=none in headers, or a policy that has been at p=none for two years with nobody reading the reports.

DMARC ties SPF and DKIM together through alignment: the domain that passes SPF or DKIM must match the domain in the visible From address. This is where most “but SPF passes!” cases fall apart. SPF can pass for your ESP’s bounce domain while your From domain gets no credit at all.

Since the Gmail and Yahoo bulk sender requirements took effect, and with Microsoft tightening enforcement for high-volume senders, a domain without DMARC sending at volume is a domain heading for the spam folder.

The Fix

1. Start with v=DMARC1; p=none; rua=mailto:[email protected]; and collect reports for two to four weeks.
2. Identify every legitimate sender in the reports and get each one aligned (custom return-path plus your own DKIM signing).
3. Move to p=quarantine; pct=25, then increase the percentage.
4. Finish at p=reject. This is the only setting that actually stops spoofing of your domain.

Verify it: Use a free DMARC report parser (dmarcian, EasyDMARC or Postmark’s free DMARC monitoring) to see alignment pass rates per source. Confirm dmarc=pass in headers.

Cause 4: You Are Failing the Mailbox Provider Requirements for Bulk Senders

Symptom: Sudden drop in Gmail or Outlook placement with no change on your side.

If you send roughly 5,000 or more messages per day to a given provider, you are treated as a bulk sender and a specific checklist applies. Missing any single item is enough to get throttled or junked.

Requirement	Detail
SPF and DKIM	Both must be present and aligned
DMARC	At minimum p=none, published on the From domain
One-click unsubscribe	List-Unsubscribe and List-Unsubscribe-Post headers, honoured within 2 days
Spam complaint rate	Stay under 0.3%, aim for under 0.1%
Valid forward and reverse DNS	Your sending IP needs a matching PTR record
TLS	Transmit over an encrypted connection
Valid RFC 5322 formatting	No spoofed From headers

Verify it: Google Postmaster Tools shows compliance status and spam rate. MXToolbox Reverse Lookup checks your PTR record. Send a test to yourself and confirm the unsubscribe link appears at the top of the message in Gmail, next to the sender name.

Cause 5: Your IP or Domain Is on a Blocklist

Symptom: Bounce messages containing text like blocked using Spamhaus, 550 5.7.1, or an SBL / CSS / DBL reference.

Blocklists react to spam traps, complaint spikes and compromised accounts. Not all blocklists carry the same weight. Spamhaus, Spamcop and Proofpoint have real impact; some obscure lists have almost none.

The Fix

• Identify which list, and read their stated reason. Do not request delisting before fixing the cause, or you will be relisted and treated less generously.
• Common causes: importing a purchased or scraped list, a compromised web form generating signups, or a hacked mailbox on your network.
• Submit a delisting request only once the underlying issue is documented and resolved.
• If you are on a shared IP pool and a neighbour caused the block, escalate to your ESP and ask to be moved.

Verify it: MXToolbox Blacklist Check, Spamhaus IP and Domain Reputation Checker, Cisco Talos Intelligence, Barracuda Reputation Lookup. Recheck 48 hours after delisting.

Cause 6: Poor List Hygiene and Spam Traps

Symptom: Hard bounce rate above 2%, and deliverability that degrades campaign after campaign.

Every invalid address you send to is a signal that you do not manage your list. Worse, abandoned addresses get recycled by mailbox providers into pristine and recycled spam traps. Hitting even a handful can tank your reputation.

The Fix

1. Run the entire list through a verification service before your next send. Remove invalid, risky and role-based addresses (info@, sales@, contact@).
2. Suppress hard bounces immediately and permanently. Suppress soft bounces after three consecutive failures.
3. Switch to confirmed opt-in for new signups, or at minimum add a CAPTCHA and a double-check on the form.
4. Never import a purchased, rented or scraped list. There is no safe way to do it.
5. Re-verify any segment that has not been emailed in more than six months, before you email it.

Verify it: Track hard bounce rate per campaign. Target under 0.5%. If it stays high after cleaning, your acquisition source is the problem, not your list.

Cause 7: You Are Emailing People Who Stopped Caring

Symptom: Open rates sliding month over month, Gmail placement worse than Outlook placement, and a big chunk of your list that has not opened in a year.

Engagement is the single strongest signal in modern filtering. Gmail in particular builds a per-recipient model. If most of your recipients ignore you, new messages get filed away automatically, including for the people who do want them.

The Fix

• Segment by recency of engagement: 0 to 30 days, 31 to 90, 91 to 180, 180+.
• Send your regular volume only to the 0 to 90 day group while you recover.
• Run a short win-back sequence (two or three emails) to the 180+ group, then suppress everyone who does not respond. Do not keep them “just in case”.
• Reduce frequency for low-engagement segments rather than increasing it.
• Add a preference centre so people can downgrade instead of hitting Spam.

Verify it: Google Postmaster Tools Domain Reputation should move from Low or Medium toward High within three to six weeks of consistent engaged-only sending. Background reading: https://kickbox.com.

Cause 8: Volume Spikes and No Warm-Up

Symptom: Deliverability collapses right after a migration to a new ESP, a new domain, or a big one-off campaign.

Sending 50,000 emails from a domain that sent 200 last week looks exactly like a compromised account. Filters throttle first and ask questions later.

The Fix

• Warm up new domains and dedicated IPs over four to six weeks, roughly doubling volume every two to three days.
• Start warm-up with your most engaged contacts, by mailbox provider. Warm Gmail, Outlook and Yahoo separately.
• Keep a predictable cadence. Regular Tuesday sends beat unpredictable bursts.
• After an ESP migration, expect a temporary dip even with perfect setup. Plan for it, do not panic-send more.

Verify it: Monitor deferrals and 4xx responses in your ESP logs during warm-up. Rising deferrals mean you are moving too fast. Hold volume flat for a few days.

Cause 9: Content and Formatting That Triggers Filters

Symptom: mail-tester score below 7, with points deducted for content rather than authentication.

Filters no longer work off a simple banned-word list, but certain patterns still add up. It is the combination that hurts, not one word.

Trigger	Fix
One giant image, almost no text	Keep a healthy text-to-image balance and always include a plain text alternative
Link shorteners (bit.ly and similar)	Use full URLs on domains you control
ALL CAPS SUBJECT LINES, excessive !!!, $$$	Write subject lines like a human, under 60 characters
Attachments, especially .zip, .exe, .html	Link to a hosted file instead
Broken HTML, inline scripts, hidden text	Validate the HTML, remove anything invisible to the reader
No physical address, no unsubscribe link	Both are legally required in most markets and expected by filters
Missing or duplicated plain text part	Send proper multipart/alternative messages

Verify it: mail-tester.com gives you a line-by-line breakdown of content penalties. Aim for 9 or 10 out of 10 with no content deductions.

Cause 10: A Shared or Unauthenticated Tracking Domain

Symptom: Authentication passes, content is clean, and you still land in Spam.

Every click-tracking link in your email points at a domain. If that domain is your ESP’s shared tracking domain, you inherit the reputation of every other customer using it, including the bad ones. Filters resolve and score those links.

The Fix

• Set up a custom tracking domain on a subdomain of your own (for example links.yourdomain.com).
• Serve it over HTTPS with a valid certificate. An expired certificate on a tracking domain is a placement killer.
• Check that the images and landing pages you link to are not hosted on flagged domains.
• Avoid redirect chains that pass through third-party domains.

Verify it: Check your tracking domain and every linked domain in Google Safe Browsing and in URL blocklist checkers such as URLVoid or Spamhaus DBL.

Cause 11: Everything Ships From One Domain

Symptom: A single bad marketing campaign takes your password resets and invoices down with it.

Transactional email, marketing email and cold outreach have completely different risk profiles. Mixing them on one domain means the riskiest stream sets the reputation for all of them.

The Fix

• Use subdomains: news.yourdomain.com for marketing, mail.yourdomain.com or notify.yourdomain.com for transactional.
• Keep cold outreach on a separate domain entirely, never on your primary corporate domain.
• Authenticate each subdomain independently, with DMARC inherited from the organisational domain policy.
• Never send business mail from a free address like @gmail.com or @yahoo.com. DMARC policies at those providers will cause outright rejection.

Verify it: Track domain reputation per subdomain in Google Postmaster Tools. Each one gets its own entry, which is exactly what you want.

Free Tools to Diagnose and Verify

Tool	Use it for	Frequency
Google Postmaster Tools	Spam rate, domain and IP reputation, compliance status	Weekly
Microsoft SNDS and JMRP	Outlook and Hotmail complaint data	Weekly
mail-tester.com	Content, authentication and configuration scoring	Every new template
MXToolbox	SPF, DKIM, DMARC, PTR and blocklist lookups	On change, then monthly
Gmail “Show original”	Instant PASS/FAIL on all three protocols	Every campaign
DMARC report parser (free tier)	Alignment failures by sending source	Weekly during rollout
Seed list / inbox placement test	Actual folder placement per provider	Before each major campaign

A 30-Day Recovery Plan

Week 1: Stop the Bleeding

• Fix SPF, DKIM and DMARC. Confirm all three pass and align.
• Add one-click unsubscribe headers if missing.
• Run a blocklist check and start any delisting requests.
• Pause sends to anyone inactive for more than 180 days.

Week 2: Clean and Segment

• Verify the full list and purge invalid addresses.
• Build engagement segments.
• Set up a custom tracking domain over HTTPS.
• Enrol in Google Postmaster Tools and Microsoft SNDS.

Week 3: Rebuild Signals

• Send only to your 0 to 30 day engaged segment, at a steady cadence.
• Test every template in mail-tester before it ships.
• Watch spam rate daily. Keep it under 0.1%.

Week 4: Expand Carefully

• Add the 31 to 90 day segment. Increase volume gradually.
• Run a win-back sequence to the dormant group, then suppress non-responders.
• Move DMARC from p=none toward p=quarantine.
• Run an inbox placement test and compare against your baseline.

Frequently Asked Questions

What is a good email deliverability rate?

At least 85% inbox placement is acceptable, and 98% to 99% is the target for a well-managed programme. Anything below 70% signals a serious reputation or authentication problem that needs immediate attention. Note that this is inbox placement, not the delivery rate your ESP reports.

What does email deliverability actually mean?

Email deliverability is the ability of your messages to reach the recipient’s primary inbox rather than the spam folder, a promotions tab, or nowhere at all. It depends on authentication, sender reputation, list quality, recipient engagement and message content.

Why do my emails land in spam in Gmail but not in Outlook?

Gmail weighs per-recipient engagement heavily, so a list full of inactive Gmail contacts will hurt Gmail placement specifically. Outlook and Microsoft 365 lean more on IP reputation and complaint feedback loops. If placement differs sharply by provider, fix engagement for Gmail and check SNDS data for Microsoft.

How long does it take to recover from low email deliverability?

With authentication fixed and clean, engaged-only sending, most senders see measurable improvement in three to six weeks. A domain that has been blocklisted or has burned reputation over years can take two to three months of disciplined sending to fully recover.

Does buying an email list ever work?

No. Purchased lists contain spam traps, invalid addresses and people who never consented. A single import can put you on a blocklist and permanently damage a domain’s reputation. It is the fastest known way to destroy deliverability.

Do I really need DMARC if SPF and DKIM already pass?

Yes. SPF and DKIM can both pass for domains that are not the one your recipient sees in the From field. DMARC enforces alignment between them and your visible sender domain, and it is now a baseline requirement for bulk senders at Gmail, Yahoo and Microsoft.

How often should I clean my email list?

Verify new signups at the point of collection, suppress hard bounces immediately, and run a full list verification every three to six months. Any segment that has been dormant for six months or more should be verified before you send to it again.

Key Takeaway

Low email deliverability is a symptom, never the disease. Work through the eleven causes in order, verify each fix with the free tool listed rather than assuming it worked, and then keep monitoring. The senders who consistently reach the inbox are not the ones with the cleverest subject lines. They are the ones who authenticate properly, mail only people who want to hear from them, and check their numbers every single week.