Email deliverability is a critical but often misunderstood aspect of email marketing. To help clarify the key concepts, this glossary provides in-depth definitions and explanations of essential email deliverability terms, from authentication protocols to engagement metrics. Whether you're new to email marketing or a seasoned professional, understanding these terms is crucial for optimizing your campaigns and ensuring your messages reach the inbox.
Authentication
Email authentication refers to the various protocols used to verify the identity of an email sender and prevent domain spoofing. The main authentication standards are:
SPF (Sender Policy Framework)
SPF is a DNS record that specifies which IP addresses are authorized to send email on behalf of a domain. It helps prevent spammers from spoofing your domain by allowing receiving servers to check if an email originates from an authorized source. SPF is checked against the envelope sender (MAIL FROM or Return-Path) domain, not the From address the recipient sees, and a record may use at most 10 DNS lookups.
example.com IN TXT "v=spf1 ip4:192.0.2.0/24 ip4:198.51.100.123 -all"
This SPF record allows emails to be sent from any IP in the 192.0.2.0/24 range and the specific IP 198.51.100.123. The -all at the end indicates a hard fail for any other source.
DKIM (DomainKeys Identified Mail)
DKIM is a cryptographic authentication method that allows the receiver to check that an email was indeed sent and authorized by the owner of the domain. It uses a private key to digitally sign selected headers and the body of the email, and the receiver verifies the signature with a public key published in the domain's DNS.
DMARC (Domain-based Message Authentication, Reporting & Conformance)
DMARC builds upon SPF and DKIM by allowing domain owners to specify how receiving servers should handle emails that fail DMARC. A message passes if SPF or DKIM passes and the passing domain aligns with the From domain; one is enough. It also provides a way for receivers to report back to the sender about the actions taken.
A DMARC policy can be set to one of three modes:
- None - No action is taken on failed messages, but reports are still sent.
- Quarantine - Failed messages are marked as suspicious and may be sent to the spam folder.
- Reject - Failed messages are outright refused by the receiving server.
Sender Reputation
Sender reputation refers to a sending IP or domain's perceived trustworthiness in the eyes of ISPs (Internet Service Providers) and email providers. It's a key factor in deliverability, as poor reputation can lead to emails being marked as spam or rejected altogether.
Factors Affecting Sender Reputation
| Factor | Description |
|---|---|
| Spam Complaints | High complaint rates indicate recipients are marking your emails as spam, hurting your reputation. |
| Spam Trap Hits | Sending to spam traps (honeypot addresses) is a strong negative signal and can quickly degrade reputation. |
| Engagement | Positive engagement like opens and clicks can improve reputation, while a lack of engagement or high bounce rates can hurt it. |
| List Hygiene | Maintaining a clean list by removing invalid and inactive addresses helps preserve good reputation. |
| Infrastructure | Having proper authentication (SPF, DKIM, DMARC) and a clean IP with no blacklistings boosts reputation. |
Monitoring Sender Reputation
It's crucial to regularly monitor your sender reputation across various providers and blacklists. Some popular reputation lookup tools include:
By proactively monitoring your reputation and addressing any issues, you can maintain high deliverability and ensure your emails reach subscriber inboxes.
Engagement Metrics
Engagement metrics measure how recipients interact with your emails. These metrics not only provide valuable insights into campaign performance but also increasingly factor into deliverability, with major providers using engagement to determine inbox placement.
Key Engagement Metrics
The percentage of recipients who opened your email out of the total delivered. While a useful measure of subject line effectiveness and overall engagement, open rates can be skewed by image caching and Apple's Mail Privacy Protection.
The percentage of recipients who clicked a link in your email out of the total delivered. Click rate measures the effectiveness of your content and call-to-action in driving engagement.
The number of recipients who completed a desired action like making a purchase or signing up for an event. Conversions are the ultimate measure of an email campaign's success and ROI.
The number of recipients who opted out of future emails by clicking an unsubscribe link. High unsubscribe rates can indicate issues with email content, frequency, or targeting.
The percentage of recipients who marked your email as spam out of the total delivered. High complaint rates are extremely damaging to sender reputation and deliverability.
Tracking Engagement Metrics
Most email service providers (ESPs) and marketing platforms offer built-in tools for tracking engagement metrics. To ensure accurate tracking:
- Implement proper email authentication (SPF, DKIM, DMARC) so more of your mail reaches the inbox and your figures reflect real engagement.
- Use unique UTM parameters in your email links for granular tracking of clicks and conversions.
- Integrate your ESP with web analytics tools like Google Analytics for end-to-end funnel visibility.
- Set up a custom tracking domain and map your ESP's tracking links to it to maintain brand consistency and avoid triggering link warnings.
Best Practice
Regularly monitor your engagement metrics and compare them to industry benchmarks. Identify trends and test optimizations to continually improve performance.Deliverability Troubleshooting
Even with a solid foundation of authentication and reputation, deliverability issues can still arise. Here are some common problems and troubleshooting steps:
A sharp decline in open rates (on a list where opens were previously stable) could indicate a deliverability problem like increased spam foldering or blocking. Analyze reputation data for any red flags, and check for recent spikes in bounces, complaints, or spam trap hits.
Possible causes:
- Broken email authentication
- IP or domain blacklisting
- Sending to stale list with many inactive or invalid addresses
If mail reaches most providers but bounces or lands in spam at one domain, the problem is usually specific to that provider's view of you. Read the bounce messages for that domain closely, as they often name the reason or link to the provider's postmaster page.
Possible causes:
- Your IP or domain is listed on a block list that provider uses
- Poor reputation with that provider (check Google Postmaster Tools or Microsoft SNDS)
- Sending too much too quickly, causing the provider to defer or throttle your mail
Conclusion
Authentication, sender reputation and engagement are closely linked: authentication proves who you are, reputation reflects how recipients have treated your mail, and engagement shapes that reputation over time. Knowing these terms makes it easier to read deliverability reports and to find the cause when something goes wrong. Test your emails before you send them and review your reputation data regularly to catch problems early.