Modern business communication is based on email. Every day, there are over 360 billion emails sent across the world. Some email delivery failures will surely occur with that volume. Diagnosing and solving the root causes may be frustrating but is important for any business that relies on email outreach.

This is a comprehensive guide on why your email may not reach the intended recipient. We’ll offer actionable tips to fix each of the possible issue areas. Even startups and companies focused on newer technologies must learn how to master email deliverability best practices if they want to communicate reliably with their customers.

What is Email Delivery Failure?

An email delivery failure occurs whenever an email does not make it to the recipient’s inbox. Instead, it may be bounced, discarded, or filtered by the email infrastructure before it reaches the mailbox it was intended for.

Senders typically receive automated bounce-back notifications when delivery failures occur. Reviewing these non-delivery receipts (NDRs) provides valuable clues for diagnosing the root cause. Startups that rely on transactional or promotional email communications should consider implementing an email API service for developers to increase delivery rates and gain more control over email flows.

Why Email Delivery Matters

Every day, billions of business and personal emails traverse the internet as an indispensable communication channel. Major email providers, such as Gmail and Outlook, use advanced filters to find spam and malware and protect their users.

Preventing email delivery failures translates directly to higher customer engagement, sales, and revenue. For startups and tech companies, reliable access to inboxes is essential for growth.

Top 10 Reasons Emails Fail To Deliver

#1: Strict Sender Reputation Policies

Major email providers like Gmail, Outlook, and Yahoo use sophisticated algorithms to evaluate a sender’s trustworthiness and past mailing behaviors.

For startups and businesses focused on rapid customer acquisition, cold outreach emails often trigger email delivery issues. Mailbox providers suspect untrustworthy senders and limit inbox access to protect their users.

Gmail and Yahoo’s algorithms factor in things like:

• Email authentication protocols in place (SPF, DKIM, DMARC)
• Complaint and spam report rates
• Consistent sending volumes and positive engagement metrics
• Inbox placement rates history

Falling short in any area prompts delivery throttling or filtering to the spam folder.

How to Improve Your Sender Reputation

Fortunately, sender reputation issues are fixable over time by adopting email best practices:

• Implement email authentication techniques
• Monitor complaint rates and ensure list quality
• Send consistent volumes to demonstrate good habits
• Prioritize relevant, engaging email content

As you demonstrate good habits, your sender score will improve. Consider warm up strategies when launching new campaigns to avoid triggering reputation filters.

#2: High Complaint Rates

ISPs track user spam complaints closely to identify untrustworthy senders. A major spike in complaints is the fastest way to sabotage deliverability.

Gmail, Outlook, Yahoo, and other big providers maintain complaint rate thresholds. Crossing the maximum tolerance for spam reports triggers automatic filtering, blocking, or blacklisting actions.

How complaints happen:

• Recipients click “Report Spam”
• Too many unsubscribe requests
• Users directly report unwanted emails

A single disgruntled recipient can spark a complaint flood, resulting in immediate deliverability declines. For any sender, getting above the 0.4% complaint threshold results in severe inbox placement drops.

How to Reduce Complaints

The best defense is subscribing to feedback loops (FBL) to identify and remove unhappy recipients. Feedback loops provide visibility into which addresses are complaining so you can take corrective actions.

Additionally, dial up relevance in email content and offers to delight subscribers. Make unsubscribing easy to allow recipients to opt out gracefully.

#3: Improper Email Infrastructure Setups

Many companies maintain their own on-premise email servers or attempt a “do-it-yourself” cloud email infrastructure. However, without proper IP warming, domain authentication, and security controls in place, deliverability suffers greatly.

Spammers typically don’t invest in proper infrastructure. So when ISPs detect misconfigurations, they penalize sender through throttling or filtering.

Common email infrastructure pitfalls:

• No SPF/DKIM/DMARC authentication
• Using low-reputation legacy IP addresses
• Insecure open relays and open proxies
• Lacking proper postmaster and abuse mailboxes

How to Fix Email Infrastructure Issues

Either create technical departments within your company for sending email or outsource your email delivery infrastructure to specialists. DIY setups always aim at saving costs, but they usually face email deliverability issues.

For in-house options, legitimate IP addresses are a good long term investment, email authentication protocols should be implemented, security best practices should be followed, and new IPs should be warmed up gradually.

Alternatively, use an email service provider to take care of authentication and sender reputation for you. For example, sending through SendGrid, Mailgun, Mailchimp, UniOne, etc., will fix email delivery issues with optimized infrastructure and setup.

#4: List Quality Issues

Bad contact data is a prevalent but preventable cause of email failures. Even with perfect technical delivery setups in place, messages can’t reach recipients if addresses are invalid or no longer used.

List decay also causes failures as subscribers change jobs or email providers and forget to notify senders. Typical data quality issues include:

• Inaccurately entered email addresses
• Typos or formatting mistakes
• Outdated records as subscribers move on
• Shared inboxes (like info@, sales@, etc.)

Catching list quality problems early is critical. With each bad address you attempt to mail, your sender reputation suffers.

How to Improve List Quality

Use an API service for verifying email addresses to tidy up your lists. For instance, SendGrid’s address validator tools instantly move invalid formats, find typos, and get rid of spam traps; their address validator tools are also available.

Also, give subscription confirmation and profile update links so subscribers can keep their details up to date. Periodically seek permission renewals to identify lapsed or fake addresses.

Finally, limit sending to generic shared inboxes like info@ or sales@. These inboxes often overflow, leading to missed emails or delivery failures. Where possible, obtain individual employee addresses.

#5: Invalid Email Addresses

Hard bounces occur whenever an attempt is made to deliver a message to a nonexistent mailbox.

Some common causes of invalid addresses are:

• Simple typos ([email protected])
• Mailing list corruption or data entry errors
• Fake email addresses entered only to access gated content
• Churned addresses after a domain goes out of business

Even with a perfect sender reputation, attempting delivery to an invalid mailbox results in an immediate failure or triggers a bounce. Too many hard bounces damage your standing with ISPs.

How to Fix Invalid Addresses

First, implement email address validation both during initial data capture and in batch scrubbing of your list. Tools like UniOne’s validator spot typos and identify disposable email addresses.

Secondly, take advantage of feedback loops and bounce notifications. These alerts supply the invalid addresses triggering failures, so you can remove them from your lists.

Maintaining data hygiene helps you avoid sending dirty lists to good addresses as collateral damage.

#6: Full Inboxes or Mailbox Quotas

Reaching a subscriber’s inbox space limit can generate a “mailbox full” error. Legacy enterprise systems can allocate only 100 MB or even less. With modern attachment sizes, it’s easy to overwhelm limited consumer inboxes too.

Quota-exceeding messages indicate the recipient’s email server won’t accept more data. The system pushes back on your messages until some space is cleaned.

How to Avoid Overflowing Inboxes

When sending marketing campaigns, include options to download content from a web portal. Link to the attachments available instead of embedding them.

For transactional email, alert recipients of quota issues and suggest steps to access and manage their inbox. Think about automatically raising limits and alerting your own users when they approach capacity if you have mailbox size control.

#7: Overzealous Spam Filters

It’s essential to have these spam filters to block malicious emails, but they also tend to get legitimate communications blocked as well. Complex rules are used to assign spam scores in filters. Messages with high risk content or from sources with bad sender reputations are filtered.

Popular reasons of false-positive spam flags are:

• Email contains spammy keywords or suspicious links
• Insecure attachments (like executable files) trigger security alerts
• Message originates from a low-reputation IP address
• No sender email authentication system in place

How to Avoid Spam Filters

Invest in spam testing tools to audit your emails before sending. Services like SendGrid’s Spam Checker simulate leading ISP filters to preview spam risk.

Avoid risky keywords frequently seen in spam messages. Attachments, images, and links should deploy over secure connections like HTTPS.

Implement SPF, DKIM, and DMARC authentication to verify your domain. Finally, monitor sender reputation and stay off blocklists.

#8: Blocklisting

Blocklists operate like blacklist databases. They catalog IP addresses and domains known for malicious sending that ISPs should preemptively filter.

Once labeled as spammers, blocklisted senders struggle greatly to reach inboxes. Their mail gets routed straight to the junk folder or rejected entirely.

Common causes of blocklisting include:

• Spikes in spam complaints
• Users reporting your mail as phishing
• Failure to properly authenticate domains
• Previous tenants abusing IP addresses

Blocklist removal involves filing appeals proving you’ve implemented proper controls and safeguards going forward. This process takes significant time and reputation-rebuilding efforts.

Avoiding Blocklists

Don’t purchase recycled IP addresses without researching their histories. Implement DMARC, DKIM, and SPF to confirm your domain’s legitimacy.

Monitor spam complaints closely via feedback loops. Finally, consider outsourcing email delivery infrastructure to avoid issues.

#9: Intermittent Server Outages

Both sender and recipient servers rely on cloud infrastructure with potential downtimes. Network glitches or intermittent service disruptions can impede emailing between otherwise healthy systems.

Causes of potential server issues are:

• Routine data center maintenance
• Infrastructure migration activities
• Power or network interruptions
• Software upgrades and patches
• Resource utilization spikes

How to Troubleshoot Server Downtimes

Check system health status pages and schedules for maintenance windows. Review the interruption history and opt in for status notifications.

For on-premise infrastructure, ensure robust disaster recovery protections are in place with failover capabilities.

When addressing recipients, provide guidance to check spam folders for any missing messages after more significant outages. Most servers queue messages during downtime to retry delivery later.

#10: Network Connectivity Issues

The pathway between sender and recipient servers depends on intermediary networks. Communication breakdowns along this email delivery chain disrupt message transit.

Connectivity issues that hamper email delivery:

• Network hardware component failures
• DNS lookup failures
• Border gateway protocol routing issues
• DDoS attacks overwhelming the infrastructure
• Network link congestion or latency

How to Address Network Issues

If you find that you can’t access your website directly, reach out to your hosting provider, cloud vendor, or email service provider for help in resolving an infrastructure problem out of your hands.

Communicate delays with recipients, give inbox checking tips, and offer an alternative communication channel while you are in transit.

In situations of regularly occurring network problems, evaluate vendor SLAs and potentially diversify delivery infrastructure across providers.

Overcoming Email Delivery Challenges

Email remains the workhorse medium powering modern business. However, subpar deliverability leaves revenue opportunities trapped in spam folders.

Step one is to diagnose the root cause of email failures. Tackling the email delivery problems listed above and many more, this guide covered the spectrum of reasons that emails don’t get to inboxes.

Rapid customer acquisition demands that both startups and enterprises with large subscriber bases have to learn deliverability basics.

Over time, good investment in authentication, infrastructure, list quality and compliance practices for troubleshooting email delivery issues will result in higher inbox placement and engagement rates.