What could be the purpose of a spammer sending emails with invalid 'To' addresses and valid 'Return-Path' addresses?
Michael Ko
Co-founder & CEO, Suped
Published 15 Apr 2025
Updated 16 Aug 2025
8 min read
Email is built on a series of protocols and headers that dictate how messages are sent and received. Typically, when you send an email, it includes a To address, which is the intended recipient, and a Return-Path address, also known as the envelope sender or Mail Fromaddress, where bounces or non-delivery reports (NDRs) are sent. When a spammer sends an email with an invalid 'To' address but a valid 'Return-Path', it raises questions about their intent.
This configuration seems counterintuitive at first glance. Why would an attacker intentionally send an email to a non-existent recipient, knowing it will bounce, yet ensure the bounce goes to a real, valid address? Understanding this unusual tactic requires a deeper dive into the mechanics of email delivery and common spamming (or phishing) strategies.
This behavior often signals an attempt to exploit email server behaviors rather than directly deliver a malicious payload to the primary recipient. It can be a sophisticated method for reconnaissance, reputation manipulation, or a form of indirect spam.
The mechanics of email headers and bounces
When an email is sent to an invalid recipient, the recipient's mail server will generate a Non-Delivery Report, or NDR. This report indicates that the email could not be delivered and is typically sent back to the address specified in the Return-Path header. This header is part of the SMTP envelope, which is separate from the From header that users typically see in their email clients.
The Return-Path is critical for bounce handling and feedback loops. According to RFC 5321, the Simple Mail Transfer Protocol standard, its primary purpose is to designate the address for error messages. When a spammer controls this address, they can manipulate the flow of these automated system messages.
This brings us to a concept known as backscatter. Backscatter occurs when a spammer sends an email to a non-existent address, but spoofs the From address to be a legitimate, third-party email. When the email bounces, the NDR is sent to the innocent third party, effectively turning them into an unwitting recipient of spam. While the scenario in question has a valid Return-Path that is also the intended target, the core mechanism of generating an NDR to a specific address remains.
Example of a manipulated email header
Return-Path: <valid-target@example.com>
Delivered-To: invalid-recipient@example.net
Received: from attacker.com ([192.0.2.1])
by mail.example.com with ESMTPSA id ABCDEF
for <invalid-recipient@example.net>;
Thu, 1 Jan 2024 12:00:00 -0000
From: "Generic Sender" <sender@example.org>
To: <invalid-recipient@example.net>
Subject: Your exciting offer!
Why spammers use this deceptive tactic
One primary reason a spammer would use this technique is for list harvesting or validating email addresses. By sending to a deliberately invalid 'To' address and setting the target's valid email as the 'Return-Path', they can confirm the 'Return-Path' address is active when the NDR arrives. This is a less common method for harvesting than traditional dictionary attacks, but it could be used for specific, targeted lists.
Another possibility is an attempt at indirect content delivery. While many NDRs do not carry the original message content, some older or misconfigured Mail Transfer Agents (MTAs) might include parts of the original email, or even the full headers, in the bounce message. If the spammer crafts a message where the malicious content is embedded in certain headers or formatted in a way that it appears in the NDR, they could bypass direct spam filters. This relies on specific MTA behaviors and is not universally effective.
This tactic might also be an attempt to exploit trust in system-generated messages. NDRs, coming from a legitimate mail server, might be perceived as more trustworthy than direct spam. If the spammer can make the NDR itself contain a call to action or a subtle malicious link within the boilerplate text or included headers, it could slip past wary users who are trained to spot direct spam, but might open a system message.
Spammer misconfiguration
It is common for spamming software to be poorly written. A simple coding error could mix up the 'To' and 'Return-Path' fields. This would result in an unintended bounce to the target, rather than a malicious scheme.
Common occurrence: Many deliverability issues stem from simple configuration mistakes, even by legitimate senders.
Lack of sophistication: Some spammers operate with minimal technical prowess, relying on brute force or outdated tools.
Deliberate strategy
The spammer might be trying to abuse the bounce mechanism for various nefarious purposes, ranging from address harvesting to indirect content delivery through NDRs.
Subtle attack vector: Exploiting system messages can sometimes bypass standard spam filters.
Reputation manipulation: Causing NDRs from a target's server can potentially impact their email sender reputation.
Impact on sender reputation and email deliverability
Even if the original email fails to deliver, the act of sending it to an invalid 'To' address with a valid 'Return-Path' can still negatively impact sender reputation. High bounce rates signal to Internet Service Providers (ISPs) and email providers that a sender's list quality is poor or that they are engaged in suspicious activities. This can lead to a sender's IP address or domain being added to a blocklist (or blacklist).
Being on an email blacklist or blocklist significantly reduces email deliverability, pushing future legitimate emails into spam folders or leading to outright rejection. This is why even seemingly harmless bounce-related spamming attempts can contribute to a sender's poor standing. ISPs use various signals, including bounce rates, to assess sender trustworthiness. Microsoft, for example, validates the From address to prevent phishing and spam, indicating how closely they monitor header information. See their guidance for further information at Microsoft's official documentation.
Additionally, if the valid Return-Path address belongs to an innocent party (as in true backscatter), their domain reputation can also suffer. Mail servers generating NDRs for these messages might eventually view the spoofed domain as a source of backscatter, impacting their own sending capabilities. This highlights the importance of proper email authentication like SPF, DKIM, and DMARC to prevent spoofing and ensure only authorized servers send on behalf of a domain.
Understanding backscatter spam
This tactic is closely related to backscatter, where innocent parties receive bounce messages for spam they didn't send. While in this specific case, the intended recipient of the bounce is also the target of the spam, the underlying principle of abusing bounce mechanisms remains similar. For more information, read our guide on what email backscatter is and how to stop it.
Protecting your domain from this type of abuse
To protect your domain from being exploited by such tactics, robust email authentication is paramount. SPF (Sender Policy Framework) helps prevent spammers from using your domain in the Mail From (Return-Path) address, while DKIM (DomainKeys Identified Mail) provides a digital signature for email content. DMARC (Domain-based Message Authentication, Reporting & Conformance) builds upon these, allowing domain owners to specify how receiving servers should handle emails that fail SPF or DKIM checks, and receive reports on authentication results.
Implementing a strict DMARC policy (p=reject) is the most effective way to prevent unauthorized use of your domain in the From header, which is the visible sender address. While DMARC primarily operates on the From address, SPF's alignment with the Mail From domain means that proper SPF setup can also defend against abuse of the Return-Path. Regularly monitoring your DMARC reports can help you identify if your domain is being spoofed for backscatter or other malicious activities.
Finally, maintaining a clean email list and avoiding sending to invalid addresses is crucial for your own deliverability. High bounce rates, even from legitimate sending, can trigger spam filters and lead to blocklisting. Integrating an email verification service and cleaning your lists regularly can prevent accidental bounces and protect your sender reputation. For more information, check out how spam traps work.
Header
Purpose
Impact of spammer manipulation
To
Intended recipient's address. Visible to the end-user.
If invalid, generates an NDR. Can be used for reconnaissance.
Can be set to the spam target for indirect delivery or list validation. Used in backscatter.
From
Sender's address shown in email clients. Subject to spoofing.
Often spoofed. DMARC helps authenticate this header.
Conclusion
While spammers' tactics can sometimes be bizarre, sending emails with invalid 'To' addresses and valid 'Return-Path' addresses is usually either a sign of poorly programmed spamware or a deliberate, albeit niche, strategy for list harvesting, indirect content delivery, or exploiting mail server trust. For legitimate senders, understanding these mechanisms underscores the importance of robust email authentication and diligent list hygiene to maintain a strong sender reputation and ensure email deliverability.
Views from the trenches
Best practices
Always implement SPF, DKIM, and DMARC with a quarantine or reject policy to prevent domain spoofing.
Regularly clean your email lists to remove invalid and inactive addresses and reduce bounce rates.
Monitor DMARC reports closely to detect any unauthorized sending from your domain, including backscatter attempts.
Educate your team on identifying suspicious bounce messages, as some may contain malicious links.
Keep your mail server software updated to patch vulnerabilities that spammers might exploit for NDR abuse.
Common pitfalls
Neglecting DMARC implementation, leaving your domain vulnerable to spoofing and reputation damage.
Failing to monitor bounce rates, which can lead to high bounce rates and negatively impact sender reputation.
Not regularly cleaning email lists, increasing the risk of hitting spam traps and invalid addresses.
Assuming all system-generated messages are safe, potentially overlooking malicious content embedded in NDRs.
Using outdated mail server configurations that are more susceptible to backscatter generation.
Expert tips
Consider setting up a dedicated 'bounce' subdomain for your Return-Path to isolate its reputation from your primary sending domain.
Leverage DMARC forensic reports (RUF) to get detailed insights into messages that fail authentication, including their headers.
Implement rate limiting on your mail servers for NDRs to mitigate potential DDoS attempts via bounce messages.
For large senders, segmenting your sending IPs can help isolate any reputation issues if one IP is caught in a backscatter loop.
Review RFCs related to email headers and delivery to gain a deeper technical understanding of how these attacks are constructed.
Marketer view
Marketer from Email Geeks says that sometimes, this behavior is just a spamming application mixing up the Return-Path and To addresses, indicating a software glitch rather than a cunning plan.
2021-05-13 - Email Geeks
Expert view
Expert from Email Geeks says it is difficult to distinguish between a spammer's deliberate strategy and poorly written software without analyzing a wide variety of the emails they send.
