What are ISP practices for identifying and handling suspicious email?
Michael Ko
Co-founder & CEO, Suped
Published 1 Jun 2025
Updated 18 Aug 2025
9 min read
Internet Service Providers (ISPs) and Mailbox Providers (MBPs) play a crucial role in safeguarding email inboxes from a deluge of malicious and unwanted messages. Their sophisticated systems are constantly evolving to identify and handle suspicious email, protecting users from phishing attacks, spam, and other forms of abuse. Understanding these practices is essential for any sender aiming for consistent email deliverability.
The sheer volume of email sent daily means that manual review is impossible. Instead, ISPs rely on a combination of automated technologies, statistical analysis, and collaborative intelligence to make real-time decisions about incoming mail. These decisions determine whether an email lands in the inbox, goes to spam, or is outright rejected.
My goal here is to shed light on the complex mechanisms ISPs employ. From initial detection techniques to ongoing reputation management, knowing these practices can significantly improve your own email sending strategies and help maintain a healthy sender reputation.
How ISPs detect suspicious email
One of the first lines of defense for ISPs involves scrutinizing email authentication protocols. SPF, DKIM, and DMARC are critical for verifying the sender's legitimacy and ensuring that the email has not been tampered with in transit. ISPs check these records rigorously, and any failure or misconfiguration can immediately flag an email as suspicious. For instance, a DMARC verification failed error often indicates a problem with how the email is authenticated, leading to potential delivery issues.
Beyond authentication, ISPs use advanced content filtering technologies. These filters analyze various aspects of an email, including its subject line, body text, links, and attachments, for patterns commonly associated with spam or phishing. Machine learning algorithms are constantly trained on vast datasets of legitimate and malicious emails, allowing them to adapt to new threats and identify even subtle anomalies. This continuous learning helps filter out sophisticated attacks that might otherwise bypass simpler rule-based systems.
Threat intelligence and blocklists (or blacklists) also play a significant role. ISPs subscribe to various public and private blocklists that compile lists of IP addresses and domains known to send spam, phishing, or malware. If an email originates from an IP or domain listed on one of these blocklists, it is highly likely to be flagged as suspicious or outright rejected. Understanding how email blacklists actually work is crucial for maintaining good deliverability.
User feedback, through spam reports, is another vital signal. When a user marks an email as spam, ISPs record this information and use it to refine their filters and assign reputation scores to senders. A high volume of spam complaints from recipients can quickly damage a sender's reputation, leading to more aggressive filtering for future emails.
Missing or incorrect records lead to flags or rejection.
Content filtering
Analyzing email body, links, and attachments for spam indicators.
Poorly crafted emails or malicious content can be blocked.
Threat intelligence & blocklists
Checking IP and domain reputation against known spam sources.
Inclusion on a blocklist means immediate rejection or spam folding.
User spam reports
Feedback from recipients marking emails as spam.
High complaint rates severely damage sender reputation.
Handling suspicious email: immediate actions
Once an ISP identifies an email as suspicious, it employs various handling mechanisms. The most common action is to quarantine the email, typically by placing it in the recipient's spam or junk folder. This allows the user to review it in case of a false positive, but it prevents the email from reaching the primary inbox where it could cause harm. In some cases, especially for highly malicious emails or those from severely compromised senders, the email may be outright rejected or dropped without delivery to protect the user entirely.
Another strategy ISPs use is rate limiting. If a sender's IP address begins sending a sudden, unusually high volume of email, or if a significant portion of their email is being flagged, ISPs might temporarily restrict the number of emails they accept from that IP. This slows down potential spam campaigns and gives the ISP time to further analyze the traffic. Persistent issues can lead to permanent blocks. For example, knowing what happens when your IP gets blocklisted can help you understand the severity of such actions.
ISPs also leverage specialized 'spam traps' (or honeypots). These are email addresses that are intentionally set up to catch spam and should never receive legitimate mail. If a sender sends email to a spam trap, it's a clear indication that their mailing list is compromised or poorly managed, leading to a significant negative impact on their sender reputation. For more details, consider reading about how to identify email spam traps.
Best practices for senders
Monitor delivery: Regularly check your email sending logs for bounces and deferrals.
Maintain clean lists: Remove inactive or bouncing addresses to avoid spam traps.
Respond to feedback: Act quickly on spam complaints and feedback loop data.
Proactive measures and collaborative efforts
Beyond reactive measures, ISPs engage in proactive efforts to improve email security. One key mechanism is the use of feedback loops (FBLs). FBLs allow legitimate senders to receive reports when their subscribers mark an email as spam. This data is invaluable for senders to identify problematic campaigns, clean their mailing lists, and improve their overall sending practices. ISPs often require senders to register for their FBLs to maintain good standing.
Collaboration across the industry is also vital. ISPs, security vendors, and other stakeholders share threat intelligence to quickly disseminate information about new spam techniques, phishing campaigns, and malware distribution. Organizations like the Messaging, Malware and Mobile Anti-Abuse Working Group (M3AAWG) facilitate this information exchange, helping to create a united front against email abuse. You can find anti-phishing best practices they publish online.
Furthermore, ISPs actively manage and monitor sender reputation. They track metrics like spam complaint rates, bounce rates, spam trap hits, and engagement signals (opens, clicks). A strong, consistent sender reputation is the cornerstone of good deliverability, while a poor reputation means emails are more likely to be filtered or blocked. Maintaining a positive email domain reputation is a continuous process that involves adherence to best practices and constant vigilance.
ISP responsibilities
Filtering threats: Protecting users from spam, phishing, and malware.
Maintaining infrastructure: Ensuring secure and efficient email delivery systems.
Sharing intelligence: Collaborating with other providers on threat data.
The foundation of modern email security rests heavily on domain authentication protocols: SPF, DKIM, and DMARC. ISPs rely on these to verify that an email truly originates from the claimed sender and has not been forged or altered. SPF (Sender Policy Framework) allows domain owners to publish which mail servers are authorized to send email on their behalf. DKIM (DomainKeys Identified Mail) provides a way to digitally sign emails, ensuring content integrity and sender identity. DMARC (Domain-based Message Authentication, Reporting, & Conformance) builds on SPF and DKIM, instructing receiving mail servers how to handle emails that fail authentication checks and providing valuable feedback to senders.
ISPs strictly enforce DMARC policies set by domain owners. A domain with a DMARC policy set to 'quarantine' or 'reject' effectively tells ISPs what to do with unauthenticated emails purporting to be from that domain. This level of control is crucial for combating spoofing and phishing, allowing ISPs to confidently filter out fraudulent messages that fail to align with the sender's stated policy. The Federal Trade Commission (FTC) provides useful guidance on recognizing phishing scams.
The proper implementation of these authentication methods directly impacts your email deliverability and sender reputation. ISPs heavily weigh these signals in their filtering decisions. A well-configured DMARC record, for example, signals to an ISP that you are serious about email security and helps them trust the legitimacy of your mail. Conversely, a lack of these records or misconfigurations can lead to emails being flagged as suspicious, even if they are legitimate. This is a primary reason why emails fail to reach the inbox.
Actively monitor DMARC reports to identify authentication failures and potential spoofing attempts.
Consistently clean your email lists to remove inactive users and reduce the risk of hitting spam traps.
Ensure all outgoing emails are properly authenticated with SPF, DKIM, and a DMARC policy.
Collaborate with your ESP to understand and improve your sender reputation metrics.
Common pitfalls
Ignoring DMARC aggregate reports, missing crucial insights into authentication failures and potential abuse.
Failing to implement a DMARC policy, leaving your domain vulnerable to spoofing and phishing.
Continuing to send to unengaged or inactive subscribers, leading to higher spam complaints and poor reputation.
Not reacting to feedback loop (FBL) complaints, which causes ISPs to increasingly filter your mail.
Expert tips
Expert from Email Geeks says: You could add a specific header to messages you find suspect, then inform mailbox providers that this header indicates spam. This can help them filter more effectively.
Marketer from Email Geeks says: IP blocks are challenging because they require manual intervention to remediate. Focusing on the spam folder concept might be better.
Marketer from Email Geeks says: Some very large mailbox providers do publish outbound IPs that they have low confidence in, implicitly telling other ISPs they can block them.
Expert from Email Geeks says: It would be beneficial if there were a standardized header with universal values for indicating suspicious email, simplifying cross-ISP communication.
Marketer view
Marketer from Email Geeks says: In the past, some ISPs published outbound IP pools designated for suspicious mail, coordinating with other ISPs to place such mail directly into the spam folder. This provided useful signals for borderline cases.
2021-09-30 - Email Geeks
Marketer view
Marketer from Email Geeks says: Some providers still operate this way, although I haven't directly received instructions to put emails into the spam folder, only assurances that blocking certain IPs for known garbage mail is understood.
2021-09-30 - Email Geeks
Wrapping up
The efforts of ISPs to identify and handle suspicious email are multifaceted and continually evolving. They combine technical authentication, sophisticated content filtering, real-time threat intelligence, and crucial user feedback to protect their customers. For email senders, this means that deliverability isn't just about sending mail, but about sending trusted mail.
Adhering to best practices, such as proper email authentication with SPF, DKIM, and DMARC, maintaining clean mailing lists, and monitoring your sender reputation, is paramount. By aligning your sending practices with ISP expectations, you not only improve your deliverability but also contribute to a safer email ecosystem for everyone.
The landscape of email security is dynamic, with new threats emerging regularly. Continuous vigilance, understanding ISP mechanisms, and proactive engagement with email deliverability best practices will ensure your messages consistently reach their intended recipients, bypassing the spam folder and avoiding outright rejection.
