Email Deliverability Troubleshooting: Diagnosing and Fixing Inbox Placement Issues

When Emails Stop Landing in the Inbox

Deliverability problems are rarely sudden. They build over time: a gradual decline in open rates, a growing number of bounces, complaints from recipients who never received your message, or a sudden spike in spam folder placement.

This guide walks through diagnosing the cause and fixing it.

Step 1: Identify the Symptom

Before troubleshooting, clarify what is happening.

Emails bouncing. You receive bounce-back messages (NDRs). The email was rejected outright. See Understanding Bounce Codes.

Emails landing in spam/junk. The email was delivered but placed in the spam folder. Recipients do not see it unless they check spam.

Emails disappearing. No bounce, no spam folder placement. The email seems to vanish. This can indicate greylisting, silent filtering or delayed delivery.

Low open rates. Open rates dropped significantly compared to historical averages. This may indicate spam folder placement, deliverability issues, or simply unengaged subscribers.

Specific providers affected. Problems only with Gmail, only with Outlook/Microsoft 365, or only with Yahoo. Provider-specific issues have provider-specific causes.

Step 2: Check Authentication

Authentication failures are the most common cause of deliverability problems. Check these first.

SPF (Sender Policy Framework)

SPF tells receiving servers which IP addresses are authorised to send email for your domain.

How to check:

Look up your SPF record:

dig TXT example.com

Or use an online SPF checker (MXToolbox, dmarcian, EasyDMARC).

Common SPF problems:

No SPF record. Without SPF, receiving servers cannot verify your sending IPs. Add one.

SPF record exceeds 10 DNS lookups. SPF has a 10-lookup limit. Each "include:" directive is a lookup. If you use multiple sending services (ESP, CRM, transactional email, cold email tool), you can hit this limit. Fix: consolidate services, use IP addresses instead of includes where possible, or flatten your SPF record.

Missing sending service. If you added a new sending tool and did not add it to your SPF record, its emails fail SPF. Check that every service sending on your behalf is listed.

Syntax errors. A typo in the SPF record causes the entire record to fail. Validate with an SPF checker.

See SPF, DKIM, and DMARC Explained.

DKIM (DomainKeys Identified Mail)

DKIM adds a cryptographic signature to your emails, verifying the message was not altered in transit.

How to check:

Send a test email to a Gmail address. Open it, click "Show original," and look for "DKIM: PASS."

Or use a DKIM checker tool with your selector and domain.

Common DKIM problems:

DKIM not configured. Many sending services require you to add DKIM DNS records during setup. If you skipped this step, your emails are unsigned.

Key rotation issues. DKIM keys should be rotated periodically. If a key is revoked before the new one is active, signatures fail.

Selector mismatch. Each sending service uses a different DKIM selector. Ensure each service's selector has a corresponding DNS record.

Message modification. If a forwarding service or mailing list modifies the email body after signing, the DKIM signature becomes invalid. This is a known issue with mailing lists and forwarding rules.

DMARC (Domain-based Message Authentication, Reporting and Conformance)

DMARC tells receiving servers what to do with emails that fail SPF and DKIM: none (monitor), quarantine (spam) or reject (block).

How to check:

dig TXT _dmarc.example.com

Common DMARC problems:

No DMARC record. Without DMARC, mailbox providers decide what to do with unauthenticated email. Gmail and Yahoo now require DMARC for bulk senders.

Policy too aggressive too soon. Moving from p=none to p=reject before fixing all authentication issues causes legitimate emails to be rejected. Ramp gradually: none, quarantine (pct=10, 25, 50, 100), reject (pct=10, 25, 50, 100).

Not monitoring DMARC reports. DMARC aggregate reports (RUA) tell you which IPs are sending as your domain and whether they pass or fail. Without monitoring, you cannot see problems.

Step 3: Check Blacklists

If your sending IP or domain is on a blacklist (also called blocklist or DNSBL), many receiving servers will reject your email.

How to check:

Use MXToolbox Blacklist Check or MultiRBL.valli.org. Enter your sending IP address and domain.

Common blacklists:

• Spamhaus (SBL, XBL, PBL, DBL). The most impactful. Being on Spamhaus SBL or DBL will block delivery to a large percentage of recipients.
• Barracuda (BRBL).
• Spamcop.
• SORBS.
• CBL (Composite Blocking List, part of Spamhaus XBL).

Why you got listed:

• Sending to spam traps. See Spam Traps Explained.
• High complaint rates (recipients marking your email as spam).
• Sending to a large number of invalid addresses.
• Compromised account sending spam.
• Shared IP with a bad sender (if using a shared sending IP).

How to get delisted:

1. Identify and fix the root cause. Do not request removal until you have stopped whatever caused the listing.
2. Submit a delisting request. Each blacklist has its own process. Spamhaus requires you to submit a removal request through their website.
3. Wait. Some blacklists (CBL, SORBS) automatically delist after a period without further abuse. Spamhaus reviews requests manually.
4. Monitor. After delisting, monitor to ensure you are not relisted.

See Understanding Blacklists.

Step 4: Check Your Sending Infrastructure

IP reputation

Your sending IP address has a reputation score. Mailbox providers track how much spam comes from each IP.

How to check:

• Google Postmaster Tools (for Gmail delivery). Shows domain and IP reputation.
• Microsoft SNDS (for Outlook/Hotmail delivery).
• Sender Score (senderscore.org). Scores your IP 0-100.

Shared vs dedicated IP:

If you use a shared IP (common with ESPs on lower plans), another sender's bad behaviour can affect your delivery. If you see deliverability issues and use a shared IP, ask your ESP about the IP's reputation or upgrade to a dedicated IP.

If you use a dedicated IP, your reputation is entirely yours. New dedicated IPs start with no reputation and need warming. See Email Warm-Up Strategies.

Sending volume patterns

Sudden spikes in sending volume trigger spam filters.

Problem pattern: Normally sending 1,000 emails/week, then suddenly sending 50,000 in one day (list import, product launch, catching up on backlog).

Fix: Ramp sending volume gradually. If you need to increase volume, do so over days or weeks, not overnight.

Infrastructure configuration

Reverse DNS (PTR record). Your sending IP should have a PTR record that matches your sending domain. Without it, many servers reject email.

TLS. Email should be sent over TLS (encrypted connection). Most modern servers support it, but misconfigured servers may fall back to unencrypted delivery, which some providers penalise.

Step 5: Check Your List Quality

Bad list quality is the most common root cause of deliverability problems.

Signs of list quality issues

• Bounce rate above 2%.
• Spam complaint rate above 0.1%.
• Low engagement (open rates below 10%).
• High percentage of unknown or invalid addresses.

Common list problems

Old lists. Lists that have not been cleaned in months or years contain invalid addresses, recycled spam traps and disengaged subscribers.

Purchased lists. Bought or rented lists consistently cause deliverability problems. They contain spam traps, invalid addresses and people who never consented to your email.

No verification on collection. If you collect emails without double opt-in or at least syntax validation, you accumulate typos, fake addresses and malicious entries.

No sunset policy. Subscribers who have not opened or clicked in 6-12 months drag down engagement metrics and may include recycled spam traps.

Fixing list quality

1. Run your list through a verification service. Remove invalid, disposable and risky addresses. See Best Email Verification Services.
2. Extract and deduplicate. Upload your list to Email Extractor to remove duplicates and standardise formatting.
3. Implement a sunset policy. Remove or suppress contacts who have not engaged in 6-12 months.
4. Segment by engagement. Send to engaged contacts more frequently. Send to disengaged contacts less often or run a re-engagement campaign.
5. Add verification at the point of collection. Real-time verification on forms. Double opt-in for newsletters.

See How to Clean an Email List and List Decay Explained.

Step 6: Check Your Content

Content filtering has become less significant than authentication and reputation, but it still matters.

Spam trigger patterns

Subject lines:

• All caps: "FREE OFFER INSIDE!!!!"
• Excessive punctuation: "Act Now??!!"
• Known spam phrases in combination with poor reputation: "Congratulations," "You've been selected," "Limited time."

Body content:

• Image-only emails with no text.
• Very short emails with only a link.
• URL shorteners (bit.ly, tinyurl) in cold email.
• Excessive HTML with minimal text content.
• Mismatched text and HTML links (display URL differs from actual URL).

Technical:

• Broken HTML.
• Missing plain-text version (multipart/alternative).
• Very large email size (images, attachments).

Testing content

Use tools like Mail Tester (mail-tester.com) or GlockApps to check your email against common spam filters before sending to your full list.

Step 7: Provider-Specific Issues

Gmail

Common issues:

• Tabs (Promotions, Updates). Marketing email often lands in the Promotions tab, which technically is delivery, not spam.
• Gmail Postmaster Tools shows domain reputation as "Bad" or "Low."
• Feedback loop: Gmail does not offer a traditional FBL. Use Postmaster Tools instead.

Fixes:

• Register with Google Postmaster Tools and monitor daily.
• Ensure DMARC alignment.
• Keep complaint rates below 0.1% (Gmail's threshold is strict).
• Send from a subdomain for marketing email to isolate reputation.

Microsoft (Outlook, Hotmail, Office 365)

Common issues:

• Aggressive filtering. Microsoft's SmartScreen filter is stricter than Gmail for unknown senders.
• Bulk mail folder. Similar to Gmail's Promotions tab.
• SNDS shows high complaint rates.

Fixes:

• Register with Microsoft SNDS (Smart Network Data Services).
• Submit to Microsoft's Junk Email Reporting Program.
• Ensure reverse DNS is correctly configured.
• Avoid sending from newly created domains without warm-up.

Yahoo/AOL

Common issues:

• CFL (Complaint Feedback Loop). Yahoo provides a robust feedback loop. If you are not processing these reports, you are not removing complainers.
• Strict authentication requirements.

Fixes:

• Register for Yahoo's CFL and process reports automatically.
• Ensure SPF, DKIM and DMARC all pass.

Step 8: Monitor and Prevent

Ongoing monitoring

Daily:

• Bounce rates by campaign.
• Spam complaint rates.
• Delivery rates.

Weekly:

• Google Postmaster Tools dashboard.
• Blacklist monitoring (automated alerts).
• Engagement trends (open rate, click rate by segment).

Monthly:

• List hygiene audit (growth, decay, engagement distribution).
• Infrastructure review (authentication, DNS records).
• Sender score trend.

Prevention

Authenticate everything. SPF, DKIM and DMARC on every sending domain.

Verify before sending. Run every list through verification before the first send. See Email Verification vs Validation.

Warm up new infrastructure. New IPs, new domains and new sending services need gradual volume ramp.

Segment by engagement. Do not treat all subscribers the same. Active subscribers get more email; inactive subscribers get less.

Process bounces and complaints immediately. Never re-send to an address that bounced or complained.

Clean regularly. Quarterly list verification. Monthly engagement-based suppression. See Automating Data Cleaning Workflows.

Troubleshooting Decision Tree

1. Are emails bouncing? Check bounce codes. Hard bounce: remove address. Soft bounce: retry, then investigate.
2. Are emails going to spam? Check authentication (SPF, DKIM, DMARC). Check blacklists. Check content.
3. Is one provider affected? Check that provider's postmaster tools and specific requirements.
4. Did volume change recently? Sending spike may have triggered filters. Throttle back.
5. Did you change sending infrastructure? New IP or domain needs warm-up.
6. Did you import a new list? List quality may be the problem. Verify and clean.
7. Have complaint rates risen? Check for content issues, frequency issues or list quality issues.
8. Is this a new domain? Warm up gradually. See Email Warm-Up Strategies.

Related Guides

• Sender Reputation Explained
• SPF, DKIM, and DMARC Explained
• Spam Traps Explained
• Understanding Blacklists
• Email Warm-Up Strategies
• Deliverability Checklist