Email Deliverability Issues: Causes and How to Fix Them

A campaign can report as sent while a substantial share of its messages never reaches a visible inbox. That makes email deliverability issues an operations problem before they're a copywriting problem. Diagnose them through measurable layers: identity, complaints, and list quality, then change the part that's failing.

Table of Contents

• Match Each Symptom to Its Root Cause
  • Capture the evidence before changing anything
• Why a Sent Email Can Still Go Unseen
  • Interpret response against actual visibility
• Set Up SPF, DKIM and DMARC Correctly
  • Audit the sending inventory before enforcement
• Keep Complaint Rates Under the Thresholds
  • Monitor the segment that caused the change
• Clean and Verify Your List Before Each Send
  • Filter for the intended workflow
• How to Fix Deliverability Step by Step
  • Contain the failure before rebuilding volume
  • Resume through controlled tests
• Decide When to Scale Sending Volume
  • Put verification between discovery and sending

Match Each Symptom to Its Root Cause

Your campaign reports as sent, but replies barely move. Before rewriting the subject line, establish whether recipients had a reasonable chance to see the message. In Sinch Mailjet's 2025 deliverability report, 48% of respondents identified staying out of spam as their biggest email-deliverability challenge.

The table gives you a starting hypothesis, not a verdict. A provider-specific failure, for example, can involve authentication, reputation, or receiving policies, so inspect the actual response before changing DNS.

Deliverability symptoms, causes and first checks

Separate your bounce rate from inbox placement. A rejection produces a delivery failure you can inspect, while a message accepted into spam may not produce a bounce at all. Open tracking also isn't conclusive evidence of human attention.

Capture the evidence before changing anything

Export the affected campaign's delivery events, receiving domains, list sources, and sending identities. Preserve rejection text rather than reducing every failure to a generic failed status.

A fix applied to the wrong cause wastes time and can damage reputation further. Find the row closest to your campaign and record its first check before making changes.

Why a Sent Email Can Still Go Unseen

Primary inbox, spam, and missing are different outcomes. Validity's 2025 Email Deliverability Benchmark reported that, worldwide during 2024, 83.5% of legitimate marketing emails reached recipients' primary inboxes, 6.7% reached spam, and 9.8% were missing, meaning blocked, rejected, or otherwise absent from both destinations.

A sent event often describes what your sending platform did, not where the recipient eventually found the message. Even a completed SMTP handoff doesn't guarantee visible inbox placement. Think of a parcel signed for at the depot but left in the wrong locker: the courier's log records progress, but the recipient still doesn't have it.

The outcome map makes that distinction explicit.

Interpret response against actual visibility

Under those benchmark conditions, a hypothetical mailing of 100,000 legitimate messages would produce approximately 83,500 inbox placements, 6,700 spam placements, and 9,800 missing messages. That's an illustration of the reported distribution, not a forecast for your B2B list.

The operational consequences are concrete: lost prospecting coverage, campaign comparisons built on unequal visibility, and uncertainty about whether recipients rejected the offer or never encountered it. Your domain reputation belongs in that investigation alongside campaign performance.

Diagnostic rule: Don't judge message quality using send counts alone. Establish where the messages went.

For a marketer-oriented companion resource, Class A Media explains ways to improve your email inbox placement. Use placement evidence to estimate the audience that could reasonably respond, then compare observed replies with your own historical response from visible mail before rewriting copy.

Set Up SPF, DKIM and DMARC Correctly

Treat authentication like passport control. SPF identifies approved issuing offices, DKIM supplies a tamper-evident seal, and DMARC tells the receiving system what policy to apply when identity checks fail.

The analogy helps separate authorization, message integrity, and enforcement.

Google's email sender guidelines, effective from February 2024, require SPF or DKIM for all senders to personal Gmail accounts, while bulk senders must use SPF, DKIM, and DMARC and meet unsubscribe and spam-rate requirements. Don't confuse the minimum for general sending with the stricter bulk-sending requirements.

DMARC also checks alignment. The domain authenticated through SPF or DKIM must align with the visible From domain, either exactly or through an allowed organizational-domain relationship. An authentication pass for an unrelated service domain isn't enough.

SPF, DKIM and DMARC at a glance

Audit the sending inventory before enforcement

Inventory every service sending as your organization: marketing automation, CRM sequences, billing, support, and application notifications. Test each source separately because fixing the campaign platform won't repair a forgotten transactional sender.

Watch for these configuration mistakes:

• Duplicate SPF records: Consolidate authorizations into a single SPF record for the relevant domain.
• Missing sending service: Confirm that a newly connected tool's authorization is included.
• Stale DKIM publication: Match the active signing selector to the published public key.
• Premature DMARC enforcement: Review legitimate failures before applying a reject policy.

This video offers another explanation of the authentication components.

Start DMARC in monitoring mode and inspect aggregate reports through normal sending cycles. Fix legitimate sources, then tighten enforcement deliberately, rather than using a stronger policy to compensate for an incomplete inventory.

Finally, send a test from every production identity. Inspect the received message's authentication results and confirm SPF, DKIM, and DMARC pass, with alignment satisfied.

Keep Complaint Rates Under the Thresholds

Irrelevant targeting, stale contacts, misleading subjects, and difficult unsubscribe paths give recipients reasons to report spam. Mailbox providers can then treat the sending identity as lower-reputation traffic and apply stronger filtering or enforcement. A technically valid address doesn't protect you from an unwanted message.

Gmail's sender FAQ advises keeping user-reported spam below 0.1% and preventing it from reaching 0.3%; bulk senders above 0.3% can lose access to delivery mitigations, with eligibility restored only after seven consecutive days below that level.

Complaint rate thresholds for bulk sending

The count column illustrates the arithmetic using a common denominator. For actual Gmail monitoring, use the provider's reported rate and definition rather than assuming your campaign platform calculates an equivalent metric.

Monitor the segment that caused the change

Calculate available complaint rates daily by campaign, segment, receiving provider, and sending identity. Use Google Postmaster Tools for Gmail visibility and your sending platform's complaint events where available. An account-wide average can conceal a problematic acquisition source.

Check the recipient experience alongside the rate:

• Subject-to-body match: Deliver what the subject promises, without disguising a sales pitch as an existing conversation.
• Recognizable sender: Use a name consistent with the brand or person the recipient expects.
• Working exit: Test the visible unsubscribe link and the one-click unsubscribe headers on bulk marketing mail.

Treat the warning zone as a reason to investigate, not as unused capacity you're entitled to consume.

Before rolling out a copy change, compare subject-line variants on appropriately limited segments and examine complaints alongside replies. A subject that wins attention but attracts spam reports isn't a useful winner.

Set campaign alerts at the operating target rather than waiting for the upper boundary. Recovery eligibility isn't a promise that inbox placement will immediately return to normal, and elapsed time alone isn't permission to restart unchanged traffic.

Clean and Verify Your List Before Each Send

A contact list is like a pantry: items with missing labels or questionable freshness shouldn't go straight onto the table. An address can look plausible and still be unsuitable for a campaign. Syntax validation, mailbox verification, and permission to contact are separate checks.

Imported records deserve particular scrutiny because their collection history and previous suppression events may be missing. Role mailboxes and free-mail addresses need contextual handling, too: they aren't automatically invalid, but they may not fit a named-contact B2B workflow.

Filter for the intended workflow

Before calling our Find LinkedIn URL by Email lookup, remove role addresses such as info@ and support@, plus addresses on free mail domains, to avoid a charge for records that cannot match. That's a lookup-specific filter, not a claim that those mailboxes can't receive email.

For campaign preparation, retain list source and collection context, verify the remaining candidates, and quarantine unresolved records. If you're deciding how to separate acquisition sources, AI Optimization Services offers a useful way to compare mailing list types.

Use bulk email verification as a controlled import stage, not as a cleanup job after the campaign has already failed. Store the verification status and check time with the contact so downstream automations don't treat an unchecked address as approved.

Suppression rules by event

Apply these rules centrally across marketing lists, rather than letting a later import reactivate a suppressed contact. Keep essential transactional messaging separate from marketing subscription logic, with its own documented handling.

Verification also doesn't create authorization to use personal information. The customer is responsible for a lawful basis for the lookup, for storing the personal email address, and for every message sent to it.

Import gate: No imported address enters a live campaign without a verification status and a suppression check.

That rule requires an explicit decision for uncertain results. Don't convert unknown or catch-all outcomes into approval just because the campaign needs more contacts.

How to Fix Deliverability Step by Step

In Mailgun's 2025 State of Email Deliverability report, nearly 80% of senders who changed practices because of new sender requirements updated their email authentication. Identity is a sensible starting point, but fixing DNS won't remove unwanted recipients or repair targeting.

Use this flow as the high-level remediation map.

Contain the failure before rebuilding volume

1. Pause the affected traffic. Stop sends to the implicated segments and apply complaint, unsubscribe, and hard-bounce suppressions. Don't pause a campaign while leaving its follow-up automation active, because those messages continue the same problem.
2. Locate the failing identity or source. Compare complaints, bounces, rejection text, and placement by campaign and receiving provider. The common slip is averaging everything together and missing a failure isolated to a particular domain or imported list.
3. Reverify quarantined records. Remove failed addresses from campaign eligibility and hold unresolved outcomes for review. Reimporting the same file without preserving suppression history recreates the incident.
4. Make a deliberate decision about disengaged contacts. Remove them from routine sends, or use a limited re-permission sequence where you still have an appropriate basis to contact them. Don't use a broad re-engagement blast as a rescue tactic while the sending identity is already struggling.

Resume through controlled tests

1. Restart with the most engaged eligible audience. Increase volume gradually only while provider responses and recipient signals remain healthy. Restarting at the previous full volume after a pause defeats the purpose of containment.
2. Test receiving providers separately. Send production-equivalent messages to seed inboxes across the providers that matter to your audience. Testing only Gmail leaves Outlook failures invisible, and a successful seed result is a useful diagnostic signal, not proof that every recipient will receive inbox placement.
3. Review recovery over a sustained observation window. Compare post-change complaints, hard bounces, placement, and meaningful engagement with the pre-change baseline. Don't bundle a new list source, sender identity, and copy overhaul into the same restart, because you'll struggle to identify which change helped or hurt.

Assign an owner to each action and record when it reached production. DNS publication, suppression deployment, and the first reduced-volume send are different events, so mark them separately in your incident notes.

For the recipient-experience work, Sprints & Sneakers provides additional guidance to reduce email complaint rates. Use that alongside technical remediation rather than treating content changes as a substitute for identity checks.

The success criterion isn't that the sending platform stops displaying errors. It's that the affected providers accept the mail appropriately, recipient complaints remain controlled, and the cleaned audience produces credible engagement without another volume-driven relapse.

Decide When to Scale Sending Volume

Scaling is a go-or-hold decision, not a reward for completing a warm-up calendar. Increase traffic only when the evidence supports the next batch. A clean DNS audit alone doesn't establish that the audience wants the messages.

Use this release checklist before increasing volume:

• Complaint stability: Provider-reported complaints remain in the target zone throughout the recovery observation window.
• List improvement: Hard bounces have fallen after cleaning when compared across equivalent list sources.
• Identity coverage: Every production sending identity passes authentication with alignment.
• Provider placement: Seed tests reach inboxes at the major providers represented in your audience.
• Suppression integrity: Reimports and enrichment jobs can't reactivate excluded marketing contacts.

Hold if any item fails. Return to the matching diagnostic check instead of compensating with a new domain or pushing extra volume through the provider that still accepts it.

Put verification between discovery and sending

A finder produces candidate contact information; it doesn't necessarily establish deliverability. Our Find Personal Email API returns the address alone without a deliverability check, while the Email Verifier API can independently check it before sending at 2 credits per address. That's one implementation option within the workflow, not a replacement for authentication or complaint monitoring.

The pipeline order should be explicit: discovery returns a candidate, verification determines campaign eligibility, suppression checks enforce exclusions, and delivery logs reveal the outcome. Keep those states separate in your CRM or orchestration system.

Measure hard-bounce performance before and after adding verification, using comparable cohorts. Otherwise, a change in acquisition source can look like a verification improvement, or conceal one.

Run the checklist against your next campaign before raising its send limit. Scale only when every item passes, and retain the evidence so the next decision isn't based on memory.

RichAPI offers contact discovery and independent email verification that can fit into an existing data pipeline. Visit RichAPI to add a verification gate before campaign entry, then use your bounce and complaint logs to measure whether the workflow improves list quality.