How to Improve Email Deliverability: 10 Fixes That Keep You Out of the Spam Folder

You hit send on a campaign, the dashboard says “delivered 99.2%”, and yet replies are flat, opens are down and your sales team swears the emails never arrived. That gap between delivered and actually seen is what email deliverability is all about.

Most articles hand you a checklist of best practices and wish you luck. This one is built differently: every fix below is paired with a detection step using free tools (Google Postmaster Tools, Mail Tester, MXToolbox, EasyDMARC, Microsoft SNDS) so you diagnose the real problem before changing anything. Guessing is how good senders break working setups.

Email deliverability vs. delivery rate: know what you are measuring

Email deliverability is the ability of your messages to reach the recipient’s main inbox instead of being blocked, bounced, filtered into spam or buried in a tab nobody opens.

Two numbers matter, and they are not the same:

• Delivery rate: the percentage of emails accepted by the receiving server (100% minus bounces). Your ESP reports this.
• Inbox placement rate: the percentage of accepted emails that land in the inbox rather than spam. Your ESP does not know this. Only seed tests, Postmaster Tools and reply/engagement data hint at it.

Metric	Healthy in 2026	Warning zone	Where to check it
Delivery rate	98% to 99%+	Below 97%	ESP dashboard
Inbox placement	95%+	Below 90%	Seed tests, Mail Tester, GlockApps
Spam complaint rate	Under 0.1%	0.3% or more (hard cap)	Google Postmaster Tools, Microsoft SNDS
Hard bounce rate	Under 1%	Above 2%	ESP dashboard
Unique open rate (B2B)	25% to 40%	Sudden 30%+ drop	ESP dashboard
Domain reputation	High	Medium, Low or Bad	Google Postmaster Tools

Your 15-minute deliverability diagnosis (do this first)

Before touching a single DNS record, gather evidence. This is the exact sequence we run for clients:

1. Send a test to Mail Tester (mail-tester.com) from your real sending platform, using a real campaign, not a blank test. Note the score and every red line.
2. Open Google Postmaster Tools and look at Domain Reputation, IP Reputation, Spam Rate and Authentication over the last 30 days. If you have not set it up, do it today: it is free and it is the only direct window into Gmail’s opinion of you.
3. Run your domain through MXToolbox for blacklist, SPF, DKIM and DMARC checks, plus your sending IP if you are on a dedicated one.
4. Check a DMARC aggregate report (EasyDMARC, Postmark’s free DMARC monitoring or your own parser) to see which sources send on your behalf and whether they pass alignment.
5. Seed test across providers: Gmail, Outlook.com, Yahoo, plus two or three corporate Microsoft 365 accounts. Record inbox, Promotions, Junk or missing for each.

Now you know whether you have an authentication problem, a reputation problem, a list problem or a content problem. Match the symptom to the fix below.

The 10 fixes, from DNS to content

1. Publish a clean SPF record (and stay under 10 DNS lookups)

SPF tells receiving servers which servers are allowed to send mail for your domain. It is the cheapest fix on this list and one of the most commonly broken. Understanding email deliverability tackles the same question from another angle.

How to detect the problem

• Mail Tester shows “SPF record not found” or “SPF check failed”.
• MXToolbox SPF lookup returns PermError: too many DNS lookups (more than 10 include mechanisms).
• Google Postmaster Tools shows SPF authentication below 100% for traffic you own.
• You find two SPF records on the same domain, which invalidates both.

How to fix it

• Keep exactly one SPF TXT record per sending domain.
• Include every legitimate sender: ESP, CRM, transactional provider, helpdesk, invoicing tool.
• Flatten or remove unused includes to get back under the 10-lookup limit.
• End with -all (hard fail) once you are confident the list is complete, or ~all while auditing.

Example: v=spf1 include:_spf.google.com include:sendgrid.net -all

2. Sign every stream with DKIM at 2048 bits

DKIM adds a cryptographic signature that proves the message was not altered and really came from your domain. Gmail, Yahoo and Microsoft all expect it from bulk senders.

How to detect the problem

• Mail Tester reports “no DKIM signature” or “DKIM signature invalid”.
• In Gmail, open a message, click Show original and look for DKIM: 'PASS' with your domain, not your ESP’s.
• Postmaster Tools DKIM authentication rate is below 95%.

How to fix it

• Enable DKIM in every tool that sends on your behalf, not just your main ESP. Forgotten invoicing and support platforms are the usual culprits.
• Use a 2048-bit key where your DNS provider allows it.
• Make sure the signing domain (d= tag) matches your visible From domain.
• Rotate keys once or twice a year and remove abandoned selectors.

3. Move DMARC from monitoring to enforcement

DMARC tells receivers what to do when SPF and DKIM fail, and it sends you reports. Since Gmail and Yahoo’s bulk sender rules (in force since February 2024) and Microsoft’s requirements for high-volume senders to Outlook.com (enforced since May 2025), a published DMARC policy is effectively mandatory for anyone sending at volume.

How to detect the problem

• MXToolbox returns “No DMARC record found”.
• You have p=none and have never read a report, so you have no idea who is spoofing you.
• Aggregate reports show failing sources you do not recognise.

How to fix it

1. Start with v=DMARC1; p=none; rua=mailto:[email protected]; and collect reports for two to four weeks.
2. Fix or remove every legitimate source that fails alignment.
3. Move to p=quarantine; pct=25, then raise the percentage gradually.
4. Finish at p=reject once reports are clean. This is what protects your brand from spoofing and what strong receivers reward.

4. Fix alignment and send from a dedicated subdomain

Authentication can pass technically while still failing DMARC because of alignment: the domain in your From header must match the DKIM signing domain or the Return-Path domain.

How to detect the problem

• Mail Tester shows SPF and DKIM as valid but flags “DMARC alignment failed”.
• The Return-Path in message headers is your ESP’s domain (for example bounces.esp.com) instead of your own.
• All streams (marketing, transactional, cold outreach, invoices) run from the same root domain.

How to fix it

• Enable custom Return-Path / custom bounce domain in your ESP so it aligns with your From domain.
• Separate streams by subdomain so a bad campaign cannot poison your password resets: news.yourdomain.com for marketing, mail.yourdomain.com for transactional, outreach.yourdomain.com for prospecting.
• Never send cold outreach from your root domain. Use a separate domain entirely if outreach volume is significant.

5. Clean the list: bounces, spam traps and dead addresses

Mailbox providers read a high bounce rate as a sign that you bought or scraped a list. Old lists also hide recycled spam traps, addresses that were real once and are now used to catch lazy senders. attentive.com makes the same point with more data.

How to detect the problem

• Hard bounce rate above 2% on any send.
• A large chunk of contacts have not opened or clicked in 6 to 12 months.
• Sudden reputation drop in Postmaster Tools right after importing a list.
• Blocklist hits on Spamhaus or SpamCop reported by MXToolbox.

How to fix it

• Run the list through a validation service before any large import and delete invalid, role-based (info@, contact@) and disposable addresses.
• Suppress hard bounces immediately and after three or four consecutive soft bounces.
• Apply a sunset policy: after 6 months of zero engagement, move contacts to a re-engagement track; after one failed attempt, stop mailing them.
• Use double opt-in for new sign-ups. It costs you list size and buys you deliverability.

6. Warm up and smooth your sending volume

Reputation is built on consistency. A domain that sends 500 emails a week then blasts 80,000 looks exactly like a compromised account.

How to detect the problem

• Postmaster Tools shows reputation falling on the days of your biggest sends.
• You get 421 or 4.7.x throttling errors and deferrals in your logs.
• New domain or new IP with no sending history behind a large campaign.

How to fix it

Ramp gradually and split large sends across several days. A workable warm-up pattern for a new domain:

Period	Daily volume	Audience
Days 1 to 7	20 to 50	Staff, partners, most engaged contacts
Week 2	100 to 300	Openers from the last 30 days
Week 3	500 to 1,500	Openers from the last 90 days
Week 4	2,000 to 5,000	Active list, segmented by provider
Week 5+	Double every 2 to 3 days	Full engaged list

Keep a stable weekly rhythm afterwards. Predictability beats intensity.

7. Engineer positive engagement signals

Gmail and Microsoft decide placement largely on what recipients do: open, reply, click, star, move out of spam (good) or delete unread, ignore, report (bad). This is where most email deliverability programmes are won or lost.

How to detect the problem

• Authentication is perfect, no blocklists, yet Gmail placement is Promotions or spam.
• Open rate trending down month over month while list size grows.
• Postmaster Tools domain reputation stuck at Medium.

How to fix it

• Mail your most engaged segment more often and your cold segment far less. Frequency should follow engagement, not the other way round.
• Send from a real person’s name at a real, monitored address. Kill every noreply@ sender you have.
• Ask questions that invite replies in the first two emails of any sequence. A reply is the strongest positive signal available.
• Segment by behaviour and lifecycle stage instead of blasting the whole database.
• Test send times per segment rather than copying generic “best time to send” advice.

8. Drive complaints below 0.1% with easy exits

Gmail requires bulk senders to stay under a 0.3% spam complaint rate and recommends staying under 0.1%. Crossing that line briefly can cost you weeks of recovery.

How to detect the problem

• Postmaster Tools Spam Rate chart spikes above 0.1%.
• Microsoft SNDS shows a red complaint rate for your IP.
• Unsubscribe rate above 0.5% on a normal campaign.

How to fix it

• Implement one-click unsubscribe (List-Unsubscribe and List-Unsubscribe-Post headers, RFC 8058). This is a requirement for bulk senders at Gmail, Yahoo and Microsoft, not a nice-to-have.
• Process unsubscribes within two days, ideally instantly.
• Put a visible unsubscribe link at the top as well as the bottom for high-frequency newsletters.
• Offer a preference centre with frequency options so “less often” becomes an alternative to “report spam”.
• Register for feedback loops (Microsoft JMRP, Yahoo CFL) and auto-suppress complainers.

9. Audit the content triggers you actually control

Filters no longer ban a list of “spammy words”, but content still matters through links, structure and how it compares to your normal patterns.

How to detect the problem

• Mail Tester deducts points for SpamAssassin rules such as HTML_IMAGE_ONLY, MISSING_MID or excessive capitals.
• One specific campaign lands in spam while others land in the inbox.
• You added a new link domain, tracking domain or shortener shortly before the drop.

How to fix it

• Maintain a real text-to-image balance and always include a plain text version.
• Use a branded tracking/click domain on a subdomain you own, never a shared ESP tracking domain and never bit.ly-style shorteners.
• Check that every linked domain is clean: one blacklisted partner link can sink a whole campaign.
• Avoid attachments in marketing mail. Link to a hosted file instead.
• Keep HTML simple and under roughly 102KB to avoid Gmail clipping.
• Do not disguise the offer. Subject lines that mismatch the body generate complaints, and complaints are what actually hurt you.

10. Monitor infrastructure, blocklists and reputation continuously

Deliverability is not a project, it is a maintenance routine. Problems are cheap to fix in week one and expensive in month three.

How to detect the problem

• MXToolbox blacklist check returns a listing on Spamhaus SBL/CSS, Barracuda or SORBS.
• Your sending IP has no PTR (reverse DNS) record, or it does not resolve back to your hostname.
• Your MX and sending hosts do not support TLS 1.2+.
• A single mailbox provider suddenly bounces everything with a policy message containing a URL that explains why.

How to fix it

• Set a weekly 10-minute routine: Postmaster Tools, blocklist check, bounce log review, complaint rate.
• Request delisting through the blocklist’s own process after fixing the root cause, never before.
• Ensure valid PTR records and TLS on every sending host (your ESP handles this on shared infrastructure).
• Read the bounce messages. SMTP rejections almost always name the exact policy you violated.
• Optional but useful: add BIMI once you reach p=quarantine or p=reject, for logo display and an extra trust signal.

Symptom to fix: quick reference

Symptom	Most likely cause	Go to fix
Bounced with “552 message rejected, unauthenticated”	SPF/DKIM/DMARC missing or misaligned	Fixes 1 to 4
Delivered but always in spam at Gmail only	Domain reputation and engagement	Fixes 5, 7, 8
High bounce rate after an import	Dirty or purchased list	Fix 5
Deferrals and throttling on big sends	Volume spike, no warm-up	Fix 6
One campaign in spam, others fine	Content, links or a listed link domain	Fix 9
Everything blocked at multiple providers overnight	Blocklist listing or compromised account	Fix 10

A 30-day deliverability recovery plan

1. Days 1 to 3: run the full diagnosis, fix SPF, DKIM and Return-Path alignment, publish DMARC at p=none with reporting.
2. Days 4 to 7: validate and segment the list, suppress bounces and anyone with zero engagement in 12 months.
3. Days 8 to 14: mail only the 30-day engaged segment, at reduced volume, with reply-friendly content. Watch Postmaster Tools daily.
4. Days 15 to 21: widen to the 90-day engaged segment, add one-click unsubscribe, launch a re-engagement campaign for the rest.
5. Days 22 to 30: raise volume progressively, move DMARC to p=quarantine, and lock in a weekly monitoring routine.

Recovery is rarely instant. Reputation is a rolling average, so expect two to six weeks before Gmail moves you from Medium back to High.

Frequently asked questions about email deliverability

What is email deliverability?

Email deliverability is the ability of your emails to reach the recipient’s primary inbox rather than being blocked, bounced or filtered into the spam folder. It depends on authentication (SPF, DKIM, DMARC), sender reputation, list quality, sending patterns, recipient engagement and message content.

What is a good deliverability rate for email?

Aim for a delivery rate of 98% to 99% and an inbox placement rate of 95% or higher, with hard bounces under 1% and spam complaints under 0.1%. Anything below 90% inbox placement means a meaningful share of your revenue-generating emails is never seen.

What is the 30/30/50 rule for cold emails?

It is a common outreach guideline: keep your sequence to around 30 to 50 emails per mailbox per day, aim for roughly 30% or more replies from engaged prospects over time, and split volume across multiple warmed mailboxes so no single domain carries more than about 50 sends a day. Interpretations vary, but the principle holds: low daily volume per mailbox, multiple warmed sending identities and heavy personalisation. Cold outreach should always run on a separate domain from your main brand.

How do I fix email deliverability problems fast?

In order: verify SPF, DKIM and DMARC pass with alignment, check for blocklist listings, remove invalid and unengaged addresses, cut volume back to your engaged segment only, add one-click unsubscribe, then rebuild volume slowly. Use Mail Tester and Google Postmaster Tools to confirm each change before moving on.

Does Google Postmaster Tools work for non-Gmail recipients?

No. Postmaster Tools only reports on traffic to Gmail and Google Workspace addresses, and it needs a certain daily volume to show data. Pair it with Microsoft SNDS and JMRP for Outlook.com and Hotmail, plus seed testing for everything else.

Should I use a dedicated IP or a shared IP?

Shared IPs are usually better below roughly 100,000 emails per month because you borrow the reputation of the pool. A dedicated IP makes sense when you send consistently high volume and want full control, but it requires disciplined warm-up and steady sending or the reputation never stabilises.

Do spam words still hurt deliverability?

Isolated words like “free” or “discount” will not get you filtered on their own. What hurts is the combination of weak authentication, poor engagement history, risky link domains and content that does not match what recipients expected when they subscribed.

Key takeaway

Strong email deliverability is not luck and it is not a single switch. It is the sum of clean authentication, an honest list, predictable volume, genuine engagement and content people asked for. Diagnose first with free tools, fix one layer at a time, and measure for a full week before judging the result.

If your campaigns are landing in spam and you want a full audit plus a content strategy your subscribers actually open, get in touch with our team.