Does the '550 5.7.1 [TCR]' error message originate from Sendmail or a Barracuda appliance?
Michael Ko
Co-founder & CEO, Suped
Published 10 Sep 2025
Updated 11 Sep 2025
7 min read
Encountering the '550 5.7.1 [TCR] Our system has detected unusual activity from your account. Contact your service provider for support' error message can be a puzzling experience. When troubleshooting, the first step is often to identify the source of the error. Is it a generic mail transfer agent (MTA) like Sendmail, or a dedicated security appliance such as a Barracuda Email Security Gateway? This particular error, with its unique '[TCR]' tag, often points to specific filtering systems rather than standard MTA rejections.
The SMTP 550 error code generally indicates a permanent failure, meaning the message could not be delivered due to an issue on the recipient's side. While the '5.7.1' subclass often suggests a security or policy violation, the 'TCR' addition provides a crucial hint about the specific technology flagging the email. Let's delve into how to pinpoint the origin and address this common, yet often misunderstood, bounce message.
The generic 550 5.7.1 SMTP error is a common bounce code that signals a permanent delivery failure due to recipient server issues. This could be anything from the recipient's mailbox being full, to security policies preventing the message from being accepted. Often, the error message provides additional details to help diagnose the specific problem. For a deeper understanding of this code, consult our guide on what a 550 5.7.1 error means.
While standard Sendmail configurations can produce 550 5.7.1 errors for various reasons, such as relay access denied or sender blacklisting, they typically do not include custom tags like '[TCR]'. Such specific identifiers are usually added by email security solutions or custom filters. This is where the distinction between a pure MTA like Sendmail and a dedicated appliance becomes important.
When you see a custom tag like '[TCR]', it's a strong indicator that a third-party security or filtering system is at play, rather than just the underlying MTA. These systems analyze email traffic for suspicious patterns, spam, and potential abuse, adding their own identifiers to rejection messages to provide more context about why an email was blocked.
Identifying the 'TCR' indicator
The '[TCR]' tag, when appended to a 550 5.7.1 error, is often associated with email security services or appliances that specialize in detecting and blocking unusual email activity. Our research, including discussions in various email deliverability forums, indicates that this specific tag has been linked to filtering solutions, especially those designed to combat spam or detect compromised accounts. The original Slack discussion confirmed that this error typically originates from a Barracuda appliance.
While MailChannels, an email security and deliverability platform, also uses the '[TCR]' tag in some of its bounce messages, it primarily flags messages for similar reasons of unusual activity. You can find more information about this error on the MailChannels support page. This correlation strongly suggests that '[TCR]' is not a standard Sendmail milter output, but rather a custom identifier from a dedicated security solution.
Example of 550 5.7.1 [TCR] error message
550 5.7.1 [TCR] Our system has detected unusual activity from your account. Contact your service provider for support
Barracuda appliances are known for their robust email security features, including advanced threat detection, spam filtering, and policy enforcement. They often sit in front of an organization's primary mail server (like Sendmail or Exchange) to preprocess incoming mail. If a Barracuda appliance detects anything suspicious, it can generate these specific error messages, even before the mail reaches the internal MTA. The presence of the '[TCR]' tag within the 550 5.7.1 error is a strong indicator of a Barracuda system identifying and rejecting the email.
How Barracuda appliances handle rejections
Barracuda Email Security Gateways play a critical role in filtering inbound and outbound email traffic. When mail flow is configured, MX records often point to the Barracuda appliance first, which then relays (or rejects) messages to the internal mail server. This architecture means that many errors, especially those related to security or policy violations, will originate from the Barracuda itself.
The presence of a higher preference MX record pointing to Barracuda means that it acts as the primary gatekeeper for incoming mail. If the appliance identifies 'unusual activity' from a sender, it will block the message and return an error like '550 5.7.1 [TCR]' directly to the sending server. This prevents the potentially malicious or unwanted email from ever reaching the internal Sendmail server. You can review Barracuda's SMTP error codes for more context.
This confirms that while a Sendmail server might be part of the recipient's email infrastructure, the '550 5.7.1 [TCR]' error is almost certainly a Barracuda-specific rejection. For more insights on how these appliances affect deliverability, consider reading about bounces from Barracuda-based domains.
Troubleshooting and resolving 550 5.7.1 [TCR] errors
Resolving a '550 5.7.1 [TCR]' error requires a multi-faceted approach, focusing on improving your email sending reputation and ensuring compliance with best practices. Since this error points to unusual activity, it's crucial to review your sending patterns and authentication.
Prevention Strategies
Implement strong email authentication protocols: Ensure your Sender Policy Framework (SPF), DomainKeys Identified Mail (DKIM), and DMARC records are correctly configured and aligned. This validates your sending identity and reduces the likelihood of being flagged.
Maintain a clean sending reputation: Avoid sending unsolicited emails, manage your mailing lists to remove inactive users, and monitor for any signs of compromise or spam activity.
Review email logs: Examine your mail server logs to confirm the bounce message and identify any preceding events that might indicate unusual sending activity, such as a compromised account.
Contact the recipient's administrator: If possible, reach out to the recipient's email administrator or IT support, providing them with the full bounce message. They can check their Barracuda logs for the exact reason for the rejection.
Adjust sending patterns: If unusual activity was detected, it might be due to a sudden increase in volume, sending to invalid addresses, or content that triggers spam filters. Address these issues to restore normal sending behavior.
For ongoing monitoring and to proactively address potential deliverability issues, using a robust DMARC reporting tool is invaluable. Suped offers a comprehensive DMARC monitoring platform with a generous free plan, helping you track authentication failures and identify sources of unauthorized email activity, which could prevent errors like '550 5.7.1 [TCR]'.
By understanding the specific context of the '[TCR]' tag, which points to security appliances like Barracuda, you can implement targeted solutions to ensure your emails reach their intended recipients. Focusing on strong authentication and maintaining a good sender reputation are key to overcoming these challenges.
Summary
The '550 5.7.1 [TCR]' error message is a clear indicator that your email was rejected by a security appliance, most likely a Barracuda Email Security Gateway, due to detected unusual activity. While a Sendmail server might be the final destination for mail, the '[TCR]' tag is a custom identifier from a dedicated filtering system, not a generic MTA. Resolving this requires a focus on email authentication, sender reputation, and understanding how these security systems operate.
Implementing and monitoring email authentication protocols like SPF, DKIM, and DMARC is crucial for preventing such rejections. Tools like Suped, with its DMARC reporting and monitoring capabilities, can provide the visibility needed to identify and address authentication failures or unusual sending patterns before they lead to widespread delivery issues. Regularly reviewing your email practices and ensuring a healthy sender reputation will significantly improve your email deliverability and reduce the incidence of these types of security-related bounce messages.
Views from the trenches
Best practices
Regularly review your DMARC reports to identify email authentication failures and potential spoofing attempts.
Ensure your SPF and DKIM records are correctly configured and aligned to prevent your emails from being flagged.
Proactively monitor your IP reputation and domain blocklist status to avoid being blacklisted by security appliances.
Adhere to email sending best practices, including list hygiene and content quality, to maintain a positive sender score.
Common pitfalls
Misinterpreting MX record priorities, which can lead to confusion about the actual mail flow and error origin.
Ignoring DMARC aggregate reports, which often contain crucial insights into how your emails are being handled.
Failing to maintain proper email list hygiene, resulting in sending to invalid or old addresses that trigger spam filters.
Not consulting vendor-specific documentation for unique error codes, leading to misdiagnosed deliverability issues.
Expert tips
Utilize a dedicated DMARC monitoring tool, like Suped, to gain clear visibility into your email authentication status.
Test SMTP connectivity and mail flow thoroughly, especially after any changes to your mail server or security configurations.
Always be aware of how intermediate filtering appliances, such as Barracuda, can impact email delivery.
Familiarize yourself with specific error code meanings provided by your email security solution vendors.
Expert view
Expert from Email Geeks says understanding your MX record priorities is crucial for tracing mail flow, especially when multiple systems are involved like Sendmail and Barracuda appliances.
2024-07-15 - Email Geeks
Expert view
Expert from Email Geeks says common 550 errors often indicate recipient server security or policy issues, making it important to check their specific error descriptions.
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