What does the Vade Secure OXSUS0001_403 error code mean and how should it be handled?
Matthew Whittaker
Co-founder & CTO, Suped
Published 29 May 2025
Updated 15 Aug 2025
6 min read
Encountering email bounce-back messages is a common frustration for anyone managing email campaigns or transactional communications. Among the various error codes, the Vade Secure OXSUS0001_403 error can be particularly puzzling because it has a dual meaning depending on the context of the email being sent. Understanding these nuances is crucial for maintaining good sender reputation and ensuring your messages reach their intended recipients.
This error code, often accompanied by a 550 5.7.1 Connection refused SMTP response, generally indicates that Vade Secure's systems have refused the email. The core challenge lies in discerning why it was refused, as the same code can point to two distinct problems. This article will break down what this error signifies and provide practical steps to address it.
Accurately diagnosing and resolving this error is key to preventing further delivery failures. Ignoring these bounce messages can lead to severe deliverability issues, including your IP or domain being added to email blacklists (or blocklists), impacting your ability to reach customers.
Understanding the OXSUS0001_403 error
The Vade Secure OXSUS0001_403 error code is documented on the Vade Secure postmaster site as having two primary interpretations. Both result in a 550 5.7.1 response, which typically signifies a permanent failure (hard bounce). This ambiguity is why it can be particularly tricky to troubleshoot.
The first meaning, particularly relevant for non-bulk mailers or transactional emails, indicates that Vade Secure could not accept your email because the format of the recipient's address provided as part of an RCPT TO command is invalid. This suggests a syntax issue with the email address itself, meaning it doesn't conform to standard email address formatting rules (e.g., missing an @ symbol or domain).
The second meaning, which is more critical for bulk mailers, states that Vade Secure could not accept your email because the message was sent to a non-existent customer/user. This is a classic hard bounce scenario, indicating that the recipient's mailbox does not exist at the destination server. For bulk senders, Vade Secure explicitly recommends treating such errors as an unsubscribe.
Why this error is challenging
The main difficulty with the OXSUS0001_403 error is that the same code is used for two very different underlying issues. This makes it challenging to automate bounce handling rules, as a simple rule might misinterpret the cause. For example, if you blindly unsubscribe users based on this code without confirming the reason, you might remove a valid recipient due to a transient formatting error.
This dual meaning is particularly problematic for email deliverability. Sending to non-existent users, often referred to as spam traps or invalid addresses, significantly harms your sender reputation. It signals to mailbox providers like Gmail and Yahoo that your list hygiene practices are poor.
Even if the issue is a simple address formatting error, repeated attempts to deliver to an incorrectly formatted address can also negatively impact your reputation. Mailbox providers expect senders to validate recipient addresses before sending, and consistent errors indicate a lack of care in your sending practices.
Scenario: Bulk mailer
Issue: The email was sent to a non-existent user. This often means the recipient address is a spam trap or an old, invalid address.
Handling the Vade Secure OXSUS0001_403 error effectively requires a nuanced approach, primarily due to its dual meaning. Your response should depend on whether you are sending bulk mail (e.g., newsletters, marketing emails) or transactional mail (e.g., password resets, order confirmations).
If sending bulk mail
Unsubscribe immediately: Vade Secure's advice for bulk mailers is clear: treat this error as an unsubscribe request. Remove the recipient from your mailing list permanently.
Prevent future sends: Do not attempt to send to this address again, even if you believe it should be active. Persistent sending to invalid addresses will damage your domain reputation.
If sending transactional mail
Verify address format: Check the email address for syntax errors. Does it follow the standard user@domain.com format? If not, correct it.
Customer contact: If the format appears correct, but you still receive this error and know the user is active, consider reaching out to the user via an alternative contact method to verify their email address.
Monitor for consistent bounces: If the error persists for a transactional email after verification, it might indicate the user's mailbox is indeed non-existent or permanently blocked, even if you believe it to be active. At that point, stop sending to the address.
Regardless of mail type, proactive email deliverability testing can help identify potential issues before they escalate. It's also vital to track your bounce rates and categorize them properly.
Beyond immediate troubleshooting, adopting long-term strategies is essential to minimize OXSUS0001_403 errors and maintain optimal email deliverability. These strategies focus on maintaining a healthy email list and ensuring proper email authentication.
Regularly clean your email lists to remove invalid, inactive, or unengaged addresses. Implementing a double opt-in process for new subscribers can significantly reduce the number of invalid email addresses entering your system from the start. You can also leverage email validation services to periodically verify the validity of your existing list.
Proper email authentication, including SPF, DKIM, and DMARC, is also crucial. While the OXSUS0001_403 error specifically relates to recipient issues, strong authentication signals to mailbox providers that your emails are legitimate, which can indirectly help your overall deliverability. This reduces the likelihood of your emails being flagged or subjected to stricter checks that might reveal address formatting issues.
Summary of action
The Vade Secure OXSUS0001_403 error, despite its single code, encompasses two distinct issues related to recipient email addresses. For bulk mailers, it indicates a non-existent user and should be treated as a hard bounce requiring immediate unsubscribe. For transactional senders, it points to an invalid address format that requires correction.
Effective management of this error is vital for maintaining good sender reputation and ensuring high email deliverability. Regularly cleaning your email lists and maintaining proper email authentication are key proactive measures that will help prevent these types of errors and ensure your emails consistently reach the inbox.
By understanding the nuances of the OXSUS0001_403 error and implementing appropriate handling strategies, you can improve your overall email program's health and efficiency. Always prioritize recipient list hygiene to avoid similar issues in the future.
Views from the trenches
Best practices
Always implement a double opt-in process to confirm subscriber intent and email address validity from the start.
Regularly clean your email lists to remove inactive, invalid, or bouncing addresses and prevent future issues.
Utilize email validation services to verify addresses before sending, especially for new or old lists, to improve data hygiene.
Segment your email sends into bulk and transactional categories to apply appropriate bounce handling rules for each.
Common pitfalls
Failing to differentiate between the two meanings of OXSUS0001_403, leading to incorrect bounce handling actions.
Continuing to send emails to addresses that consistently return permanent bounce errors, damaging sender reputation.
Not regularly cleaning email lists, which leads to higher bounce rates and increased risk of hitting spam traps.
Ignoring bounce notifications and error codes, which prevents timely identification and resolution of deliverability issues.
Expert tips
Monitor your bounce logs closely to identify patterns in OXSUS0001_403 errors and determine their underlying cause.
If the email address seems valid but you're still getting the error, verify with the user via an alternative contact method if possible.
Consider setting up automated rules to handle different types of bounces, but ensure they are flexible enough for ambiguous errors.
Engage with the Vade Secure postmaster team if you believe the error is being misclassified or if you need clarification.
Marketer view
Marketer from Email Geeks says they are seeing both deliveries and OXSUS0001_403 errors for the same recipient within a short period, highlighting the inconsistency.
2021-01-01 - Email Geeks
Marketer view
Marketer from Email Geeks says Vade Secure's support confirmed that if the mail is bulk, the customer should be unsubscribed, but if it's not bulk, it's a format error. However, differentiating between these two can be challenging.
