Which DNS records are most important for email domain validation?

DNS records like MX (Mail Exchange), SPF (Sender Policy Framework), DKIM (DomainKeys Identified Mail), and DMARC (Domain-based Message Authentication, Reporting, and Conformance) are critical. MX records tell you if a domain can receive email. SPF, DKIM, and DMARC help verify the sender's authenticity and prevent spoofing. Checking these records helps identify invalid or suspicious domains.

What is the impact of a domain being on an email blocklist?

A blocklist (or blacklist) is a database of IP addresses or domains known for sending spam or malicious content. If an email domain is on a blocklist, emails sent from or to that domain are likely to be rejected or sent directly to spam folders, severely impacting deliverability and sender reputation. Regularly checking against these lists is essential.

How can I identify problematic domains in my existing email list?

Key indicators include high bounce rates, low engagement (opens and clicks), high unsubscribe rates, and an increase in spam complaints. DNS errors like NXDOMAIN or SERVFAIL for recipient domains also signal problematic entries. Regularly monitor these metrics and conduct periodic list cleaning to maintain hygiene.

Why is double opt-in important for email list hygiene?

Double opt-in verifies that subscribers genuinely want your emails and helps filter out invalid or bot-generated addresses. It ensures that only valid, engaged recipients are added to your list, reducing bounces and improving overall list quality, which is crucial for maintaining a good sender reputation.

How can I filter and sanitize a large list of email domains using DNS and other techniques? - Technical - Email deliverability - Knowledge base

Managing large email lists comes with inherent challenges. Over time, email domains can become inactive, be repurposed, or even be used for malicious activities, leading to a significant impact on your email deliverability and sender reputation. Sending emails to these problematic domains results in higher bounce rates, increased spam complaints, and can ultimately land your domain on a blocklist (or blacklist), hindering your ability to reach legitimate inboxes.

Filtering and sanitizing these lists is not just a best practice, it is a necessity for anyone aiming for optimal inbox placement. By proactively identifying and removing problematic domains, you safeguard your sender reputation and ensure your campaigns reach their intended audience. This article will explore how to leverage DNS and other techniques to effectively clean your email domain lists.

The goal is to maintain a healthy email ecosystem, preventing your legitimate emails from being flagged as spam. Understanding the underlying mechanisms of email routing and domain validation is key to this process.

Understanding DNS in email validation

The Domain Name System (DNS) is the backbone of the internet, playing a crucial role in how emails are routed and delivered. When an email is sent, the receiving mail server performs DNS lookups to verify the sender’s domain and ensure the email is legitimate. This process involves checking various email authentication protocols, such as SPF, DKIM, and DMARC, which all rely on DNS records.

A fundamental DNS check involves looking up the Mail Exchange (MX) records for a domain. These records specify the mail servers responsible for accepting email on behalf of a domain. If a domain has no MX records, or if its MX records point to invalid or non-existent servers, it is likely that emails sent to that domain will bounce. Similarly, domains returning an NXDOMAIN (non-existent domain) or SERVFAIL (server failure) response during a DNS query indicate that the domain is not properly configured for email receipt.

Checking MX recordsbash

dig MX example.com

Beyond basic record validation, DNS is also crucial for utilizing DNS-based blocklists (DNSBLs), sometimes called blacklists. These are real-time databases of IP addresses and domains known to send spam or malicious content. By querying a DNSBL, you can quickly identify and filter out emails associated with bad actors or compromised domains. Spamhaus Domain Blocklist is a well-known example that lists domains found in spam.

Key DNS records for email

MX (Mail Exchange) records: Specifies the mail servers responsible for receiving email for a domain.
A/AAAA records: Map domain names to IP addresses, essential for server connectivity.
SPF (Sender Policy Framework) records: Authorize which mail servers can send email on behalf of a domain, published as a TXT record. This helps prevent email spoofing.
DKIM (DomainKeys Identified Mail) records: Digitally sign emails, allowing recipients to verify the email's authenticity and integrity. These are also TXT records.
DMARC (Domain-based Message Authentication, Reporting, & Conformance) records: Instruct receiving mail servers how to handle emails that fail SPF or DKIM checks. It also provides reporting on email authentication results.

Beyond basic DNS checks

While DNS checks are foundational, a comprehensive domain sanitization strategy extends beyond simple record lookups. Advanced filtering techniques are essential to tackle more subtle forms of problematic domains that can still harm your sender reputation. One significant category is disposable email domains (DEDs). These temporary domains are often used to sign up for services without revealing a primary email address, leading to low engagement and high bounce rates, as they typically expire quickly. Identifying and filtering them requires specialized lists and pattern recognition.

Another critical aspect is identifying bot-generated or suspicious email addresses. These often come from automated sign-ups or malicious scripts and can include typo domains (e.g., gamil.com instead of gmail.com) or domains with unusual character combinations. Detecting these requires heuristic analysis, pattern matching, and sometimes even behavioral analysis of sign-up sources. Domains associated with spam traps, which are email addresses specifically set up to catch spammers, are particularly damaging. Hitting a spam trap can immediately land your domain on a severe blacklist.

Leveraging external domain reputation data is also key. Various providers compile lists of domains with poor sending histories, high complaint rates, or association with malware. Integrating these external blocklists (or blacklists) into your filtering process can prevent you from sending emails to domains that are already deemed harmful or unresponsive. This multi-layered approach ensures a cleaner list and better deliverability, preventing issues before they arise.

Manual review and ad-hoc scripts

Process: Involves custom scripts to check MX records or ping domains. Manual spreadsheet analysis for anomalies.
Scalability: Limited, becomes impractical for very large lists due to time and resource demands.
Accuracy: Prone to human error, misses sophisticated threats like disposable domains.
Cost: Lower direct financial cost, but high labor cost and risk of deliverability issues.

Automated email validation services

Process: Utilizes advanced algorithms, real-time DNS checks, and proprietary databases for comprehensive validation.
Scalability: Designed to handle millions of emails efficiently, making them suitable for any list size.
Accuracy: High accuracy in identifying invalid, disposable, and problematic domains. Reduces spam trap hits.
Cost: Incurs a service fee, but offers long-term savings through improved deliverability and reduced marketing waste.

Proactive strategies for a clean list

Effective email deliverability isn't a one-time fix, it requires ongoing attention to your list hygiene. Proactive strategies are essential to prevent issues from accumulating and to maintain a healthy sender reputation over time. One of the most effective preventive measures is implementing a double opt-in process for new subscribers. This verifies that the email address is valid and that the subscriber genuinely wants to receive your emails, significantly reducing the likelihood of invalid or bot-generated entries.

Regularly monitoring engagement is another cornerstone of proactive list management. Low open rates, high bounce rates, and increased spam complaints are clear signals that parts of your list may be stale or problematic. It is important to segment your list based on engagement and consider re-engagement campaigns for inactive subscribers. If they remain unresponsive, it is often best to remove them to protect your sending metrics and overall domain reputation.

Beyond cleaning, focus on email authentication. Properly configured SPF, DKIM, and DMARC records are crucial for telling receiving mail servers that your emails are legitimate and not spoofed. These DNS records add a layer of trust, making it harder for spammers to impersonate your domain and helping your emails pass through filters without issue. Keeping these records updated is a continuous task.

Maintaining accurate and up-to-date DNS records for your sending domains is also essential. This includes ensuring that your MX records are correctly configured and that your domains are not listed on any major blocklists. Regular monitoring for blocklist (or blacklist) presence and swift remediation if listed are vital. This proactive stance ensures that your efforts in email marketing are not undermined by technical oversights.

Domain Health Indicator	Description	Impact on Deliverability	Common tools for checking
MX record validity	Ensures the domain can receive emails, indicating it's an active mail-receiving endpoint.	Directly affects deliverability; invalid MX leads to bounces.	DNS lookup tools, email validation services
SPF record configuration	Authorizes specific servers to send email on behalf of your domain.	Poorly configured SPF can cause emails to fail authentication and land in spam folders. It is a critical component for sender reputation.	SPF record checkers
DKIM record validity	Ensures email content hasn't been tampered with in transit through cryptographic signatures.	DKIM failures can lead to emails being rejected or marked as suspicious.	DKIM validators, DMARC reporting tools
DMARC policy status	Defines how recipient servers handle emails that fail SPF or DKIM alignment, and provides feedback.	Essential for brand protection and improving email trust, helps mitigate spoofing.	DMARC analyzers, DMARC reports
Blocklist presence	Indicates if your domain or IP is listed on a DNS-based blocklist due to suspicious activity.	Being on a blacklist (or blocklist) can lead to significant deliverability issues, often outright rejection. An in-depth guide to email blocklists provides more detail.	Blocklist checking tools, deliverability platforms

Implementing an effective workflow

Implementing an effective domain sanitization workflow involves a series of steps to ensure thorough cleaning and ongoing maintenance. Start by obtaining a raw list of all email domains you interact with. This could be from your CRM, marketing automation platform, or any other data source. The larger and older the list, the more likely it is to contain problematic entries.

Next, subject this raw list to a multi-layered validation process. Begin with DNS lookups for MX records to identify domains that cannot receive email. Filter out non-existent domains (NXDOMAIN) and those with server failures (SERVFAIL). Follow this with checks against known disposable email domain lists. Also, use pattern recognition and heuristic analysis to flag potential bot-generated or suspicious entries. This process will help you accurately verify your list.

Finally, integrate real-time blocklist (or blacklist) checks into your workflow. Services like Spamhaus provide domain blocklists that can identify domains associated with spam or malicious activity. Regularly cross-referencing your list against such blocklists allows you to quickly suppress problematic domains, preventing them from impacting your sender reputation. For ongoing maintenance, consider automated email validation services that can continuously clean and monitor your list, especially if you have high volume sign-ups. This continuous process is critical for long-term deliverability success.

Views from the trenches

Best practices

Implement double opt-in for all new subscribers to ensure email validity and genuine interest.

Regularly monitor your email engagement metrics like open rates and click-through rates to identify inactive domains.

Segment your email lists based on engagement levels to tailor your communication and manage inactive segments.

Utilize DNS-based blocklists and domain reputation services for real-time filtering of problematic domains.

Automate your email list cleaning process with a reputable service to handle large volumes and complex checks.

Common pitfalls

Relying solely on basic syntax validation without performing deeper DNS or content checks.

Neglecting to regularly clean inactive subscribers, leading to lower engagement and potential spam traps.

Ignoring bounce notifications, which indicate invalid or non-existent domains that harm sender reputation.

Not implementing email authentication protocols like SPF, DKIM, and DMARC for your sending domains.

Failing to review and update your domain blocklists, missing new threats or re-activated bad domains.

Expert tips

For very large, historical lists, a phased approach to cleaning can minimize disruption and identify patterns.

Consider a 'sunrise period' for new domains where you closely monitor engagement before full integration.

Use email validation services as a gatekeeper during sign-up to prevent bad data from entering your list.

Analyze DMARC reports to uncover unexpected sources sending email on behalf of your domain.

Implement a feedback loop service to automatically process unsubscribes and spam complaints efficiently.

Expert view

Expert from Email Geeks says a client sent them years of domain data for analysis to identify relevant filters, indicating the scale of data hygiene challenges.

May 13, 2022 - Email Geeks

Marketer view

Marketer from Email Geeks mentioned encountering many variations of localhost and invalid domains, highlighting the messy nature of raw lists.

May 13, 2022 - Email Geeks

A comprehensive approach to domain sanitization

Filtering and sanitizing a large list of email domains is a multifaceted task that blends technical DNS expertise with ongoing list hygiene practices. From fundamental MX record checks to sophisticated analysis for disposable and bot-generated domains, each layer of validation contributes to a healthier email ecosystem.

Proactive measures, such as implementing double opt-in and consistently monitoring engagement, are just as crucial as reactive cleaning efforts. By adopting a comprehensive approach that includes robust DNS checks, leveraging blocklists (blacklists), and maintaining strict list hygiene, you can significantly improve your email deliverability, protect your sender reputation, and ensure your messages consistently reach the inbox.