How can I prevent my domain from being blacklisted due to an infected employee's computer or scraping contact information?
Matthew Whittaker
Co-founder & CTO, Suped
Published 16 May 2025
Updated 15 Aug 2025
7 min read
As businesses increasingly rely on digital communication, the threat of having your domain blacklisted (or blocklisted) is a serious concern. A domain blocklist can severely impact your email deliverability, preventing important messages from reaching their intended recipients. Two common, yet often overlooked, culprits behind such listings are compromised employee computers and the practice of scraping contact information.
Understanding how these issues lead to blocklisting is the first step toward prevention. When an employee's computer becomes infected with malware, it can silently send out spam or malicious emails using your corporate domain, leading to your domain being flagged. Similarly, using email addresses gathered through web scraping often results in sending unsolicited emails, which quickly lands your domain on a blacklist.
In this guide, I will explore the preventative measures and best practices to safeguard your domain from these specific threats, ensuring your legitimate emails consistently reach the inbox.
One of the most insidious threats to your domain's reputation comes from within: an infected employee computer. Malware can turn a legitimate workstation into a spam bot, sending out malicious emails under your domain's name. What often happens is that your Mail Transfer Agent (MTA) logs the IP address of the originating machine within the email headers, and if that IP is compromised or dynamic (like a Wi-Fi IP), it can trigger blocklists. Even if your primary sending IPs are clean, these internal IPs showing up in headers can be problematic for some email filters.
The impact of malware on your domain
When an infected machine on your network sends out spam or malware via your email infrastructure, it can quickly lead to your domain (and associated IP addresses) being added to an email blacklist or blocklist. This severely hampers your email deliverability, as receiving servers will often reject or quarantine any email originating from your domain. Fixing this requires identifying the source of the infection, cleaning it, and then working with blocklist operators for delisting.
The hidden threat of scraped data
Another significant risk is the use of email addresses obtained through scraping. While some businesses might view this as a quick way to build a contact list, it often violates consent policies and privacy regulations. Email addresses gathered this way are typically not opt-in, leading to high bounce rates, spam complaints, and eventually, blocklisting of your domain. Many anti-spam systems are designed to detect and block emails sent to scraped lists.
Even if the volume of emails sent to scraped addresses is low, the lack of consent makes them highly susceptible to being marked as spam. Email service providers and receiving mail servers are increasingly strict about unsolicited email, regardless of its content. This practice can quickly erode your sender reputation and lead to your domain being placed on a blocklist (or blacklist), making it difficult for your legitimate emails to be delivered.
Implementing technical safeguards
To effectively prevent your domain from being blocklisted due to internal or external factors, implementing robust technical safeguards is paramount. This includes proper Mail Transfer Agent (MTA) configuration and strong email authentication protocols like DMARC, SPF, and DKIM.
Mail Transfer Agent (MTA) configuration
Your MTA is responsible for sending and receiving emails. A common issue leading to blocklistings is when internal IP addresses, such as those from an employee's Wi-Fi connection or local network, are inadvertently included in the email headers visible to the recipient server. If these internal IPs are associated with malicious activity (even if by another user on a shared public network), it can negatively impact your domain's reputation. Configure your MTA to strip out or hide these internal IPs from outbound messages.
Example MTA configuration (PowerMTA)plaintext
hide-message-source
remove-header "Received from"
Strong email authentication protocols
Implementing robust email authentication is critical. DMARC, SPF, and DKIM records help verify that emails sent from your domain are legitimate and have not been tampered with. This significantly reduces the chances of your domain being spoofed by attackers or mistakenly marked as spam.
The power of DMARC
A DMARC policy (Domain-based Message Authentication, Reporting, and Conformance) allows you to instruct receiving mail servers how to handle emails that fail SPF or DKIM checks. By moving from a p=none policy to p=quarantine or p=reject, you actively prevent unauthorized use of your domain for sending emails.
Best practices for email hygiene and security
Technical measures are crucial, but human factors and email list hygiene also play a significant role in preventing domain blocklistings. Proactive employee training and ethical list management are key to maintaining a strong sender reputation.
Employee training and security awareness
Employees are often the first line of defense, but also a potential vulnerability. Regular training on cybersecurity best practices, such as identifying phishing attempts, using strong passwords, and maintaining up-to-date antivirus software, is essential. Educate your team on the importance of not sending sensitive company emails from unsecured public Wi-Fi networks if their email client bypasses your corporate MTA. Emphasize that any suspicious activity on their work device should be reported immediately.
Email list management and consent
The foundation of good email deliverability is a clean, opt-in email list. Avoiding scraped email addresses is non-negotiable. Using contact information without explicit consent not only risks domain blacklisting (or blocklisting) but can also lead to legal repercussions under regulations like CAN-SPAM or GDPR. Focus on building your list organically through transparent signup processes. Regularly verify your email list to remove invalid or inactive addresses that could become spam traps.
Legitimate list building
Opt-in consent: Obtain clear, explicit consent from subscribers. Double opt-in is ideal.
Transparent practices: Clearly state what emails will be sent and how often.
Regular hygiene: Remove inactive subscribers and hard bounces to maintain list quality.
Problematic list building
Scraping: Automated harvesting of email addresses from websites or directories.
Purchased lists: Buying lists from third parties, which often contain unverified or spam trap addresses.
Lack of consent: Sending to individuals who have not explicitly agreed to receive your emails.
Preventing blacklisting when scraping is a challenging task because many scraping techniques are inherently designed to bypass detection, which often leads to being flagged as malicious. For more information on avoiding being blocked during web scraping, you can refer to resources on how to prevent blacklisting when scraping.
Monitoring and rapid response
Even with the best preventative measures, domain blocklistings can still occur. This makes continuous monitoring and a rapid response plan essential for mitigating damage and restoring email deliverability.
Continuous blocklist monitoring
Regularly checking your domain and IP addresses against major blacklists (or blocklists) is crucial. Early detection allows for quicker remediation, minimizing the impact on your email campaigns and operational communications. Tools that offer blocklist monitoring can provide real-time alerts. It's also beneficial to monitor your domain reputation through services like IP reputation databases.
Incident response plan
Have a clear plan in place for when a blocklisting occurs. This should include steps to identify the cause (e.g., infected machine, spamming activity, bot attacks), mitigate the issue, and request delisting. Many blocklists provide clear instructions for removal, but proving the issue has been resolved is often required. Speed is of the essence, as prolonged blocklisting can significantly damage your domain's sending reputation.
Actively monitor your mail logs for suspicious activity and unusual outbound email patterns to detect compromises early.
Implement strong DMARC policies (quarantine or reject) to prevent unauthorized use of your domain.
Train employees on email security and phishing prevention to reduce the risk of infected machines.
Maintain strictly opt-in email lists and avoid any form of scraping contact information to ensure compliance.
Configure your MTA to remove internal IP addresses from outbound email headers to prevent accidental blocklistings.
Common pitfalls
Assuming that only your main sending IP reputation matters, ignoring potential issues from internal IPs in headers.
Neglecting employee security training, making internal systems vulnerable to malware that can send spam.
Using scraped or purchased email lists, which inevitably lead to high spam complaints and blocklist listings.
Delaying the investigation and remediation process once a blocklist notification is received, worsening the reputation damage.
Misinterpreting blocklist notifications or relying on basic support, rather than investigating the root cause.
Expert tips
Regularly audit email authentication (SPF, DKIM, DMARC) to ensure proper configuration and alignment.
For large organizations, consider network segmentation to limit the blast radius of an infected machine.
Implement endpoint detection and response (EDR) solutions to proactively identify and contain malware on employee devices.
Engage with IT security teams immediately if a potential compromise related to email sending is detected.
If using cloud-based email services, understand their policies on header information and sender reputation management.
Marketer view
Marketer from Email Geeks says: I realized that if I work from a cafe using their Wi-Fi, and if that IP happens to be infected, my corporate sending domain reputation could be at risk if that IP appears in mail headers.
2020-03-11 - Email Geeks
Marketer view
Marketer from Email Geeks says: If your main outbound IP is blocked because it's sending malware, removing the client's IP from headers won't fix the core problem, as the block is based on the actual outbound server.
2020-03-11 - Email Geeks
Maintaining a healthy email reputation
Preventing your domain from being blacklisted (or blocklisted) is an ongoing effort that requires a multi-faceted approach. It combines technical configurations, robust security practices, and a deep understanding of email deliverability standards. By proactively addressing threats from infected employee computers and avoiding the use of scraped contact information, you can significantly enhance your domain's reputation and ensure your emails reach their intended audience.
Remember that a healthy domain reputation is built on trust and adherence to best practices. Continuous monitoring and a swift, informed response to any issues will help you maintain optimal email performance.
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