What could be the purpose of a spammer sending emails with invalid 'To' addresses and valid 'Return-Path' addresses?

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.

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.

Marketer from Email Geeks raises a possibility of an auto-ack spam scenario where the spammer sends mail to their own freemail account with auto-ack enabled (containing the spam content) and sets the victim as the sender.

Email marketer from Random Forum explains that using invalid 'To' addresses and valid 'Return-Path' addresses could be part of a distributed denial-of-service (DDoS) attack against email servers. By generating a high volume of bounce messages, spammers can overwhelm the server and disrupt its normal operation.

Email marketer from Email Hippo explains that spammers might send emails with invalid 'To' addresses and valid 'Return-Path' addresses to harvest valid email addresses from auto-replies and bounce messages. By monitoring the responses and bounce reports, spammers can identify active and engaged email addresses for future spam campaigns.

Email marketer from Email Marketing Forum explains that the spammer may be attempting to manipulate sender reputation systems. By sending to invalid addresses and receiving bounce messages, they might try to trick the reputation system into thinking they are a legitimate sender, as they are seemingly 'cleaning' their list.

Email marketer from StackExchange says that a spammer might use invalid 'To' addresses to test the effectiveness of anti-spam filters and identify weaknesses. By observing how the filters react to these malformed emails, they can adapt their techniques to bypass them.

Email marketer from MailerCheck shares that using invalid 'To' addresses could be a method to test email verification tools and identify which addresses are valid or invalid. Spammers might use this technique to clean their lists, identify active email addresses, and improve their targeting.

Email marketer from Security StackExchange suggests this could be a basic method of bounce address tagging. Where spammers uses it to track open rates based on auto generated 'out of office' replies, which they can use to identify the most active users in that domain, so they can focus their efforts.

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.

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.

Expert from Email Geeks shares how bounce.io used to sell advertising in bounce messages.

Expert from Email Geeks says it's difficult to distinguish between a spammer's cunning plan and poorly written spam software without a decent variety of the emails they're sending.

Expert from Email Geeks suggests spammers might rotate through sender addresses.

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.

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.

Documentation from IETF explains that invalid "To" addresses are not compliant with email protocols. Spammers exploit this by utilizing malformed addresses to create confusion in email systems, which can then be used to bypass security checks.

Documentation from Mimecast explains that the technique described could be a form of directory harvesting. By sending emails to a range of invalid addresses, spammers can identify valid email addresses when they receive bounce messages, indicating that the mailbox exists.

Documentation from RFC Editor answers that the 'Return-Path' (MAIL FROM) and 'To' (RCPT TO) addresses are part of the SMTP envelope, which dictates how the email is delivered. Spammers might manipulate these addresses to hide their identity or to exploit vulnerabilities in email servers.