Email Deliverability Issues: Common Problems and How to Fix Them

Email deliverability issues are frustrating because they happen silently. Your emails leave your outbox and either never arrive or land in spam, and nobody tells you. This guide covers the most common problems, how to recognise each one from its symptoms, and how to fix them in the order that gets results fastest.

Find your symptom

Start with what you can see. Each symptom points at one or two likely causes.

Worked example: what the bounce is telling you

Bounce messages are the most useful evidence you have, and most people never read past the first line. Two real-world patterns:

A Gmail rejection for missing authentication:

550-5.7.26 This mail has been blocked because the sender is unauthenticated.
550-5.7.26 Gmail requires all senders to authenticate with either SPF or DKIM.
550-5.7.26  Authentication results:
550-5.7.26  DKIM = did not pass
550-5.7.26  SPF [example.com] with ip: [203.0.113.55] = did not pass

Everything you need is in the last two lines. Gmail checked DKIM and found no valid signature for your domain. It checked SPF for example.com against the sending IP 203.0.113.55 and found the IP is not authorised. The fix is entirely in DNS: add the sending service to SPF and publish its DKIM key. No amount of content or list work will help until that is done.

A Microsoft rejection for a blacklist listing:

550 5.7.1 Service unavailable, Client host [203.0.113.55] blocked using Spamhaus.
To request removal from this list see https://www.spamhaus.org/query/ip/203.0.113.55 (S3140)

This one names the list and gives you the lookup URL. Authentication may be perfect. The IP is listed, and Microsoft will not accept mail from it until it is delisted. The fix is to find out why the IP was listed, stop the cause, and request removal. If the IP is shared, ask your ESP, since it is their listing to clear.

For the full catalogue of codes, see email bounce codes.

Authentication issues

Authentication problems are the most common deliverability issue and the most fixable.

SPF failures

Symptoms: rejections mentioning SPF, spam placement at providers that check SPF strictly, "not authorized" or "does not designate" in bounce text.

Causes: the SPF record does not include your sending service, exceeds the ten-lookup limit, has a syntax error, or does not exist.

Fix:

1. Check your SPF record
2. Add every service that sends as your domain
3. Flatten or consolidate if you are over the lookup limit. See SPF too many lookups
4. Correct syntax errors

DKIM failures

Symptoms: dkim=fail or dkim=none in headers, reduced deliverability despite valid SPF.

Causes: the DNS record does not match the signing configuration, the wrong selector is in use, keys were rotated without a DNS update, or signing is not enabled.

Fix:

1. Test your DKIM
2. Confirm the record matches what your sending service shows
3. Enable signing and confirm the selector

See DKIM fail for every DKIM result and its fix.

DMARC failures

Symptoms: dmarc=fail in headers even when SPF or DKIM pass, rejections from providers enforcing DMARC.

Causes: the passing check is for a different domain than your From address (an alignment failure), no DMARC record exists, or the policy was set to reject before legitimate sources were confirmed.

Fix:

1. Review your DMARC record
2. Make SPF or DKIM align with the From domain, usually by completing domain authentication in your ESP
3. Start at p=none, read the reports, then tighten

See DMARC fail and DMARC alignment explained.

Reputation issues

Reputation problems take longer to fix than authentication.

Blacklisting

Symptoms: sudden delivery failures at specific providers, bounces naming a blacklist, overall delivery rate drops.

Fix:

1. Check blacklist status
2. Identify the cause: complaints, spam trap hits, a compromised account, or a bad neighbour on a shared IP
3. Fix the cause, then request removal following each list's process
4. Monitor to prevent relisting

See the blacklist removal guide.

Low sender reputation

Symptoms: gradual decline in deliverability, more spam placement, poor reputation in provider tools.

Causes: high complaint rates over time, high bounce rates, low engagement, spam trap hits.

Fix: clean the list, improve targeting, make unsubscribing easy, send to engaged recipients first to rebuild, and allow time. Reputation recovers slowly. See email sender reputation.

Provider-specific reputation

Symptoms: good deliverability everywhere except Gmail, Outlook, or Yahoo specifically.

Diagnose: Google Postmaster Tools for Gmail, Microsoft SNDS for Outlook, and the complaint feedback loop for Yahoo.

Fix: address that provider's specific requirements, focus on engagement from those recipients, and reduce volume to them temporarily if needed. See Gmail spam issues and Outlook deliverability issues.

List quality issues

Poor list quality causes recurring problems that no DNS change will fix.

High bounce rates

Symptoms: bounce rate above 2%, many hard bounces, bounces increasing over time.

Causes: an old list with decayed addresses, a purchased or scraped list, data entry errors at collection.

Fix: remove hard bounces immediately, validate before sending, use confirmed opt-in for new subscribers, and prune unengaged addresses regularly. See the list hygiene guide.

Spam trap hits

Symptoms: blacklist listings that mention traps, sudden reputation drops with no obvious cause.

Causes: purchased lists, old lists containing recycled trap addresses, scraped addresses, inactive subscribers never removed.

Fix: remove anything that has not engaged in six months, never use purchased or scraped lists, and keep strict hygiene.

High complaint rates

Symptoms: complaint rate above 0.1%, reputation drops in provider tools, feedback loop reports arriving.

Causes: mailing people who did not opt in, sending too often, content that does not match expectations, a hard-to-find unsubscribe link.

Fix: review how addresses were collected, make unsubscribe prominent and one-click, reduce frequency or let subscribers choose, and send re-permission campaigns to uncertain segments. See email complaint rate.

Infrastructure issues

Sending IP problems

Symptoms: issues specific to one sending IP, rejections mentioning IP reputation, better results from other infrastructure.

Fix: check IP blacklist status and provider tools, address the cause, request delisting, and consider a dedicated IP for isolation, with a proper warmup.

DNS configuration errors

Symptoms: intermittent authentication failures, DNS-related bounce text, records that look right sometimes and not others.

Causes: propagation delays, TTL caching, conflicting records, zone file errors.

Fix: verify every record, allow propagation time after changes, check for duplicates (two SPF records is a classic), and test from more than one location. See DNS propagation and email.

Mail server misconfiguration

Symptoms: failures regardless of recipient provider, technical errors in bounces, authentication passing but delivery failing.

Causes: wrong SMTP configuration, missing or mismatched reverse DNS, TLS problems, a server that identifies itself incorrectly.

Fix: set a PTR record that matches the HELO name, confirm TLS works, and check the server's HELO or EHLO identity. See PTR records for email.

Throttling

Symptoms: temporary 4xx errors, "try again later", mail delayed for hours rather than rejected.

Causes: a volume spike from an unfamiliar source, low reputation, or a provider incident.

Fix: spread sends over a longer period, follow a warmup schedule for new sources, and check the provider's status before assuming the problem is yours. See email throttling explained.

Content issues

Content rarely causes deliverability problems on its own, but it can tip borderline mail.

Symptoms: specific campaigns filtered while others succeed, content-related flags in spam filter results.

Check: subject line patterns (capitals, stacked punctuation), spam trigger phrases and suspicious formatting, link destinations (shorteners, unknown domains), image-to-text ratio, and the legally required elements: a physical address, a working unsubscribe link, accurate sender identification, honest subject lines.

Fix: simplify the HTML, add a plain-text part, replace shortened links with full URLs, and test changes one at a time. See email content spam triggers.

Troubleshooting process

When you are not sure where to start, work in this order. Each step rules out the most likely cause before you spend time on the next.

1. Check authentication. SPF, DKIM, and DMARC. Fix any failure before going further.
2. Check blacklists. Run a blacklist check. If listed, fix the cause and request removal.
3. Review metrics. Bounce rate, complaint rate, engagement. Anything outside the normal range is your next lead.
4. Test delivery. Send to accounts at the major providers and read the headers. See how to test email deliverability.
5. Check provider tools. Postmaster Tools, SNDS, and your ESP's deliverability data.
6. List what changed. New sending service, DNS edit, list import, content change, volume change. The cause is usually in the last thing that changed.

Stop Worrying About Email Deliverability

The Email Deliverability Suite monitors your SPF, DKIM, DMARC, MX records and blacklist status daily. Get alerts when something breaks, before your emails stop arriving.