How to identify and handle email forging and replay attacks?
Matthew Whittaker
Co-founder & CTO, Suped
Published 12 Jun 2025
Updated 16 Aug 2025
9 min read
Email is a cornerstone of modern communication, but its inherent openness also makes it vulnerable to malicious attacks like forging and replay. These types of attacks can severely undermine trust, compromise data, and damage an organization's sender reputation.
Email forging, also known as spoofing, involves creating emails with a false sender address, making them appear to originate from a legitimate source. Replay attacks, on the other hand, involve intercepting and retransmitting valid data, often to gain unauthorized access or manipulate systems.
Understanding how these attacks work and implementing robust defense mechanisms is crucial for maintaining email security and ensuring deliverability. I'll explain how to identify and effectively handle these threats to protect your email infrastructure and recipients.
Identifying email forging (spoofing)
Email forging, commonly referred to as spoofing, is the act of sending emails with a falsified sender address. Attackers manipulate the email headers, specifically the 'From' address, to trick recipients into believing the message came from a trusted entity, such as a known colleague, a bank, or a reputable company. This tactic is a primary component of phishing campaigns, where the goal is often to illicit sensitive information or financial gain.
Identifying a forged email requires careful scrutiny beyond the display name. While a sender's name might look legitimate, the underlying email address often reveals discrepancies. Look for subtle misspellings, unusual subdomains, or completely unrelated domains that don't match the expected sender. Many email clients offer options to view the full message headers, which can provide a deeper insight into the actual sending server and authentication results.
A common sign of email forging is when an email purports to be from your own domain, but you know it wasn't sent by your legitimate systems. This form of impersonation is a serious threat to your brand's reputation and can lead to internal and external compromise. Recognizing these subtle (and sometimes not-so-subtle) signs is the first line of defense.
Deciphering replay attacks
A replay attack in the context of email involves an attacker intercepting a legitimate, authenticated email and then re-sending it to a target recipient or even multiple recipients. The danger lies in the attacker's ability to reuse valid authentication credentials or signatures that were present in the original email, making the replayed message appear authentic even if it wasn't originally intended for the new recipient or was sent outside its original context.
One common type is a DKIM replay attack, where a cybercriminal takes a legitimate, properly signed email, and then forwards or redistributes it. This exploits the trust placed in DKIM signatures. If the original email did not include mechanisms to prevent replay (like unique identifiers or timestamps that expire quickly), the replayed email could pass DKIM verification. This makes it particularly challenging to detect, as standard authentication checks might show the email as legitimate.
Replay attacks are often harder to spot than simple forging because the technical authentication (like DKIM) might actually pass. This is why a comprehensive email security strategy that goes beyond basic authentication is essential. Look for context, unexpected recipients, and unusual content to flag these sophisticated attacks. You can learn more about preventing DKIM replay attacks for deeper insights.
Preventing replay attacks with nonces and timestamps
To prevent replay attacks, including dynamic elements in your email headers or body is a strong countermeasure. A nonce (number used once) or a timestamp that changes with each email makes it difficult for an attacker to reuse a captured email. Even if intercepted, the replayed email would be flagged as invalid due to outdated or duplicated unique values.
Key authentication protocols for defense
The most effective way to combat email forging and replay attacks is through the robust implementation of email authentication protocols, namely SPF, DKIM, and DMARC. These protocols work together to verify the legitimacy of incoming emails and ensure that they haven't been tampered with in transit. If you're looking for a simple breakdown of how these protocols work, check out our guide on DMARC, SPF, and DKIM.
SPF, DKIM, and DMARC in brief
SPF (Sender Policy Framework): This DNS TXT record lists the IP addresses authorized to send email on behalf of a domain. It helps prevent spammers from sending messages with a forged 'envelope from' address. If an email comes from an unauthorized IP, SPF validation will fail, signaling a potential spoofing attempt. SPF checks on the return path, so it might pass even if the 'From' address is forged, but it will likely fail alignment with DMARC.
DKIM (DomainKeys Identified Mail): DKIM adds a digital signature to outgoing emails, allowing the receiving server to verify that the email was sent by an authorized sender and hasn't been altered in transit. A missing or invalid DKIM signature can be a red flag for both forging and replay attacks.
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 authentication (e.g., quarantine or reject them) and to receive reports on authentication failures. This protocol is crucial for enforcing your email sending policy and gaining visibility into forging and replay attempts against your domain. For more info, see our article on handling spoofed emails violating DMARC policies.
Implementing a DMARC policy with a strong enforcement setting, such as p=quarantine or p=reject, is critical. While a p=none policy provides visibility, it doesn't instruct receivers to take action against unauthenticated mail. Transitioning to a stricter policy gradually, after analyzing DMARC reports, helps prevent legitimate emails from being incorrectly blocked. See our guide on how to safely transition your DMARC policy.
Beyond implementing SPF, DKIM, and DMARC, continuous monitoring and proactive mitigation strategies are essential for identifying and handling forging and replay attacks. Even with robust authentication, sophisticated attackers can find loopholes or exploit misconfigurations. Staying vigilant is key.
Monitoring and detection
DMARC reports: Regularly review your DMARC aggregate and forensic reports. These reports provide invaluable insights into who is sending email on behalf of your domain, including unauthorized senders. Pay close attention to sources that are failing SPF or DKIM authentication, especially if they are attempting to impersonate your domain. Our guide on troubleshooting DMARC failures and DKIM replay attacks can help.
Mail logs: Monitor your mail server logs for unusual sending patterns, high bounce rates from unknown recipients, or anomalies in authentication results. These can be indicators of your domain being used for malicious purposes, even if DMARC is set to p=none. Cisco's guide provides further anti-spoofing best practices.
Recipient feedback: Educate your users and customers on how to report suspicious emails that appear to be from your domain. Their feedback can be an early warning system for widespread forging or replay attempts.
If you detect forging or replay activity against your domain, prompt action is necessary. Update your DNS records (SPF, DKIM, DMARC) immediately to block unauthorized senders. Report malicious IPs to relevant abuse desks and consider collaborating with your email service provider or IT security team to investigate further and block traffic from known malicious sources. Being listed on an email blacklist (or blocklist) can severely impact your deliverability, so proactive detection and remediation are vital. Our guide to email blocklists provides more context on these lists.
Views from the trenches
Best practices
Always implement DMARC with an enforcement policy to protect your brand against unauthorized sending.
Regularly monitor your DMARC reports for signs of email forging or replay attacks.
Educate internal teams and customers on how to identify and report suspicious emails.
Use DKIM oversigning to add random numbers or timestamps, making replay attacks harder to execute.
Stay informed about the latest email security threats and best practices.
Common pitfalls
Leaving DMARC policy at `p=none` for too long, offering no protection against spoofing.
Ignoring DMARC reports, missing critical insights into unauthorized domain usage.
Failing to update SPF records when adding new sending sources, leading to legitimate email failures.
Assuming standard email authentication (SPF/DKIM) alone is sufficient to stop all attacks.
Not having a clear incident response plan for detected email forging or replay incidents.
Expert tips
Implement a strict DMARC policy gradually, starting with `p=quarantine` and moving to `p=reject` after careful monitoring.
Use subdomains for different email purposes (e.g., marketing.yourdomain.com, transactional.yourdomain.com) to isolate potential issues.
Regularly audit your DNS records to ensure they are correctly configured and up-to-date.
Consider a forensic DMARC report analysis service for deeper insights into attack vectors.
Marketer view
A marketer from Email Geeks says they encountered a situation where SPF was passing but the sending IPs were completely unknown, which raised concerns about a potential attack using compromised instances.
2022-11-15 - Email Geeks
Marketer view
A marketer from Email Geeks says they confirmed that if SPF passes due to the return path, it would still fail DMARC alignment, which is a good indicator of email forging.
2022-11-15 - Email Geeks
Protecting your email ecosystem
Email forging and replay attacks pose significant threats to email security and deliverability. Forging aims to deceive recipients by faking the sender's identity, while replay attacks exploit valid signatures to retransmit intercepted emails. Both can lead to phishing, data breaches, and severe damage to your brand reputation.
The foundation of defense lies in correctly configuring and enforcing SPF, DKIM, and DMARC. These authentication protocols are your primary tools for verifying email legitimacy and instructing receiving servers on how to handle unauthenticated messages. Beyond technical setup, continuous monitoring of DMARC reports and vigilance against suspicious email patterns are crucial for early detection.
Proactive steps, such as implementing strong DMARC policies, educating users, and having an incident response plan, will fortify your email ecosystem. By taking these measures, you can significantly reduce your vulnerability to these attacks, protect your recipients, and safeguard your domain's sending reputation.
