Summary
Spammers exploit emails with invalid 'To' and valid 'Return-Path' addresses for a variety of reasons, including arbitrary behavior and address rotation. A key use is backscatter spam to flood recipients or hide origins. This technique enables testing email verification tools for list cleaning, harvesting addresses from auto-replies, and testing anti-spam filters. It also facilitates bounce address tagging, DDoS attacks, manipulation of sender reputation, and identity masking. Analyzing email variety is crucial to differentiate between planned attacks and software errors. Furthermore, it's related to list bombing, SMTP envelope manipulation, directory harvesting, and protocol violations. Stricter address verification, bounce message monitoring, adherence to email protocols, and enhanced server security are recommended.
Key findings
- Arbitrary Behaviour: Spammer actions and spam software are unpredictable.
- Backscatter Spam: Invalid 'To' generates bounces to valid 'Return-Path', flooding recipients or obscuring the source.
- Address Harvesting: Harvesting valid email addresses from auto-replies and bounce messages.
- Filter Testing: Testing spam filters to adapt spam techniques.
- DDoS: Potential use in DDoS attacks.
- List Bombing: Testing lists to find active addresses.
- Identity Masking: Hiding identity and complicating traceback efforts.
- Protocol Violation: Violating email protocols by creating malformed email systems.
Key considerations
- Monitor Bounce Rates: High bounce rates may indicate spam activities.
- Analyse Return Path: Investigate the return path for suspicious patterns.
- Implement Robust Filters: Utilizing solid anti-spam filters to detect and block malicious emails.
- Address Verification: Validating email addresses before sending.
- Compliance with email protocols: Ensure compliance with standards
What email marketers say
9 marketer opinions
Spammers utilize emails with invalid 'To' and valid 'Return-Path' addresses for a variety of purposes. These include backscatter spam campaigns to flood recipients or obscure the spam's origin, testing email verification tools to clean and refine target lists, harvesting valid email addresses from auto-replies, testing anti-spam filters, and conducting bounce address tagging to identify active users. They might also attempt distributed denial-of-service (DDoS) attacks or manipulate sender reputation systems. Furthermore, using this method can enable masking their identity and complicating traceback efforts. Another less sinister use is testing an auto-ack email.
Key opinions
- Backscatter Spam: Invalid 'To' addresses generate bounce messages to the valid 'Return-Path', flooding the recipient or obscuring the original spam source.
- List Cleaning: Spammers test email verification tools to identify valid addresses, refining their target lists.
- Address Harvesting: Valid email addresses are harvested from auto-replies and bounce messages.
- Filter Testing: Spammers test anti-spam filters to identify weaknesses and adapt their techniques.
- Bounce Tracking: Used for bounce address tagging to track open rates and identify active users.
- DDoS Attacks: Generate high volumes of bounce messages to overwhelm email servers.
- Reputation Manipulation: Attempt to trick reputation systems by simulating list cleaning.
- Identity Masking: Obscure true identity and complicate traceback efforts.
- Auto Ack Testing: Used to test and receive Auto Acknowledgement of Emails from servers
Key considerations
- Monitor Bounce Rates: High bounce rates may indicate spam activity or list quality issues.
- Analyze Return-Path: Investigate the 'Return-Path' address for suspicious patterns.
- Implement Anti-Spam Measures: Utilize robust anti-spam filters to detect and block malicious emails.
- Sender Reputation: Regularly check sender reputation to avoid being flagged as a spammer.
- Address Verification: Consider using email verification services to validate email addresses before sending.
- DMARC, SPF and DKIM: Consider implementing these email protocols to better authenticate your emails.
Marketer view
Email marketer from Reddit explains that this could be a form of backscatter spam, where the spammer uses an invalid 'To' address to generate bounce messages (NDRs) to a valid 'Return-Path' address. The purpose may be to flood the recipient with unwanted messages or to obscure the original source of the spam.
29 Sep 2021 - Reddit
Marketer view
Email marketer from CyberNews explains that spammers use invalid "To" and valid "Return-Path" addresses in order to mask their true identity. By causing errors in the email system, they can redirect attention away from their actual origination point, making tracking and traceback significantly harder.
15 Mar 2023 - CyberNews
What the experts say
5 expert opinions
Spammer behavior is often arbitrary, where spamware may select a from/return path from a batch of addresses or rotate through sender addresses. Distinguishing between a planned attack and poorly written spam software requires analyzing a variety of sent emails. Using invalid 'To' and valid 'Return-Path' addresses is also a common technique in list bombing, allowing spammers to test and refine their targeting. There used to be companies such as Bounce.io that would sell advertising in bounce messages - showing its possible for spammers to exploit this.
Key opinions
- Arbitrary Spam Behavior: Spam software may randomly select from/return paths, indicating unpredictable behavior.
- Address Rotation: Spammers may rotate through sender addresses to evade detection.
- Software vs. Strategy: Differentiating between spammer intent and software errors requires analysis of multiple emails.
- List Bombing: Invalid 'To' addresses used for testing and refining target lists.
- Bounce Advertising: Bounce messages are sometimes used for advertising.
Key considerations
- Analyze Email Variety: A decent variety of emails are needed to distinguish spammer intent from software errors.
- Monitor Sender Reputation: Continuously monitor sender reputation to detect potential abuse.
- Implement Robust Filters: Implement robust spam filters to identify and block malicious emails.
- Track bounceback messages: It's possible that bounceback messages are used for advertisement.
Expert view
Expert from Email Geeks shares that spammer behavior can be arbitrary and spamware often selects a from/return path from a batch of addresses when sending spam.
8 Jun 2022 - Email Geeks
Expert view
Expert from Word to the Wise explains that using invalid 'To' and valid 'Return-Path' addresses is a technique often associated with list bombing. Spammers might use this to test a list, determining which email addresses are valid and which are not, allowing them to refine their targeting in future campaigns.
12 Dec 2024 - Word to the Wise
What the documentation says
5 technical articles
Spammers exploit the use of invalid 'To' addresses with valid 'Return-Path' addresses to manipulate the SMTP envelope, hide their identity, and exploit vulnerabilities in email servers. This technique enables directory harvesting, allowing spammers to identify valid email addresses from bounce messages. Address spoofing is facilitated by creating confusion and bypassing security measures. Since invalid 'To' addresses violate email protocols, spammers create confusion to bypass security checks. This approach also circumvents standard sender verification systems, enhancing spam effectiveness by exploiting the limited scrutiny of 'To' address validity.
Key findings
- SMTP Envelope Manipulation: Spammers manipulate 'To' and 'Return-Path' addresses within the SMTP envelope to hide their identity.
- Directory Harvesting: Invalid 'To' addresses are used to identify valid email addresses through bounce messages.
- Address Spoofing: The technique facilitates address spoofing, bypassing security measures.
- Protocol Violation: Invalid 'To' addresses violate email protocols, creating confusion for security bypass.
- Verification Bypass: Standard sender verification systems are bypassed due to less stringent checks on 'To' address validity.
Key considerations
- Implement Robust Verification: Enforce rigorous verification of both 'From' and 'To' addresses to prevent spoofing.
- Monitor Bounce Messages: Analyze bounce messages for suspicious patterns indicative of directory harvesting.
- Adhere to Email Protocols: Ensure strict adherence to email protocols to prevent malformed addresses.
- Enhance Server Security: Strengthen email server security measures to mitigate exploitation of vulnerabilities.
Technical article
Documentation from Cisco Talos shares that this practice allows for bypassing standard email sender verification systems, which are usually configured to ensure the "From" address is not spoofed, but often do not check the validity of "To" addresses to the same extent. By bypassing this check, spammers can increase effectiveness.
12 Apr 2024 - Cisco Talos
Technical article
Documentation from Microsoft explains the described technique allows for address spoofing. It shares that by using a valid return path and invalid To addresses a spammer can cause confusion and potentially bypass some security measures.
15 Apr 2022 - Microsoft
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