What causes high Barracuda bounce rates after migrating to HubSpot and how to resolve them?
Matthew Whittaker
Co-founder & CTO, Suped
Published 16 Jun 2025
Updated 17 Aug 2025
8 min read
A sudden surge in email bounce rates, especially from Barracuda, right after migrating to a new platform like HubSpot can be a frustrating experience. I've seen this happen where clients transition their engaged audience, only to find a significant portion of their emails being rejected, often with a 550 permanent failure message indicating they are "blocked." This situation is common but requires a strategic approach to diagnose and resolve.
The core issue often stems from how email security gateways, such as Barracuda, perceive your sending activity from a new, previously unknown source, even if your domain remains the same. The change in the sending infrastructure (from your old ESP to HubSpot) triggers their protective mechanisms, leading to increased rejection. This is particularly challenging when the bounced contacts were highly engaged on your previous platform.
Barracuda is a prominent email security appliance widely used by businesses to protect their inboxes from spam, phishing, and malware. It employs sophisticated filtering mechanisms, including real-time blacklists (or blocklists), content analysis, and sender reputation checks, to identify and block suspicious email traffic. When you migrate your email sending to a new platform like HubSpot, even with your existing domain, Barracuda sees a significant change in the originating IP addresses. This change can raise red flags, leading to increased scrutiny and potential blocking.
One crucial aspect of Barracuda's filtering is its aggressive link following and content analysis. It will often click every link in an email to assess its safety, which can sometimes be misinterpreted as genuine engagement. This automated activity can make it difficult to distinguish real user engagement from Barracuda's own checks, potentially skewing your engagement metrics if not accounted for.
The 550 permanent failure for one or more recipients (blocked) message from Barracuda indicates that the email was outright rejected due to its internal policies. This isn't necessarily a sign of a non-existent email address, but rather a strong signal that the content, sender reputation, or IP address (or a combination) triggered a block. For specific details on this error, consult the SMTP Field Manual.
Barracuda is known for its stringent email filtering rules. A sudden influx of email from a new IP range, even if associated with a familiar domain, can be perceived as suspicious activity. This can lead to a high volume of emails being rejected, despite prior engagement from the recipients.
Common causes of Barracuda bounces after migration
The primary reason for high Barracuda bounce rates post-migration often relates to the change in sending infrastructure. HubSpot, like many large email service providers, often uses shared IP addresses, especially during the initial phases of a new account or for senders with lower volumes. If other senders on the same shared IP have poor sending practices or are on a blocklist, your emails can suffer by association. This is a common challenge when moving to a new platform.
Content-based filtering also plays a significant role. Even if your content was fine on the old platform, Barracuda's algorithms might flag certain elements when coming from a new source. This could include specific keywords, URL structures, or even the overall formatting. Any deviation from what Barracuda previously recognized as "normal" for your domain, combined with the new IP, can lead to increased filtering.
Sender reputation is heavily tied to both your sending domain and the IP address. While your domain reputation might be strong from your previous sending history, the new IP address from HubSpot will have its own reputation, which needs to be built up. Barracuda evaluates both, and if the IP reputation is low, or if there's a perceived mismatch between your established domain reputation and a new, relatively unknown IP, it can result in a block or blacklist (also known as a blocklist) entry.
Before HubSpot migration
Before migrating to HubSpot, your email sending would have been associated with your previous ESP's IP addresses and sending patterns.
Established reputation: Your IP and domain had a consistent, positive sending history, well-regarded by Barracuda and other filters.
Predictable volume: Barracuda recognized your sending volume and frequency from your old infrastructure.
Consistent authentication: Your SPF and DKIM records aligned with your previous sender.
After HubSpot migration
Moving to HubSpot means your email sending originates from a new set of IP addresses, which can trigger strict filtering.
New IP reputation: HubSpot's IP addresses may not yet have an established reputation with Barracuda for your sending domain, especially if you are on a shared IP.
Volume spike on new IP: Sending large volumes too quickly from new IPs appears suspicious. You might experience this during IP warming.
Authentication setup: Ensuring your DMARC, SPF, and DKIM records are correctly configured for HubSpot is critical.
Resolving high Barracuda bounce rates
The most critical step to resolve high Barracuda bounce rates after migrating to HubSpot is to re-evaluate and adjust your IP warming strategy. Sending too many emails too quickly from a new IP (even if shared by HubSpot) can trigger Barracuda's filters. It's often beneficial to warm up more slowly, gradually increasing your sending volume to domains protected by Barracuda. Prioritizing engagement segments helps. For more details on this process, you can explore guides on IP warming for deliverability.
Next, you need to conduct a thorough review of your email list and content. Even if contacts were engaged previously, some may have become invalid since your last send from the old platform. Validate your email list to remove any dormant or invalid addresses that could be triggering spam traps. Examine your email content for anything that might be perceived as spammy, such as excessive links, certain keywords, or poor formatting. Barracuda can be sensitive to these factors, even for legitimate mail.
Finally, direct engagement with recipients is crucial, especially for B2B domains protected by Barracuda. If possible, reach out to key contacts through alternative channels (e.g., phone, other email addresses) and ask them to whitelist your sending domain. This is often the most effective way to bypass Barracuda's filters for critical recipients. You might also encounter issues if you're experiencing general bounces from Barracuda.
Typical Barracuda Bounce Messageplaintext
550 permanent failure for one or more recipients (user@example.com:blocked)
Issue
Description
Resolution
New IP reputation
HubSpot's IP addresses (which might include Amazon SES backbone) lack a positive history with Barracuda.
Implement a slower, more deliberate IP warming strategy for Barracuda-protected domains.
Content flagging
Email content is triggering Barracuda's spam filters from the new sending source.
Review email copy, links, and images for suspicious elements. Conduct A/B testing on content to Barracuda recipients.
Domain authentication issues
Improperly configured SPF, DKIM, or DMARC records for HubSpot can weaken your sending legitimacy.
To prevent future Barracuda (or other blocklist) issues and ensure long-term email deliverability, consistent monitoring and proactive list management are essential. Keep a close eye on your bounce rates, especially for specific domains or ISPs, and segment your list based on engagement to identify and suppress unengaged contacts. Ignoring high bounce rates can severely damage your sender reputation.
Implement robust email authentication. Properly configuring SPF (Sender Policy Framework), DKIM (DomainKeys Identified Mail), and DMARC (Domain-based Message Authentication, Reporting & Conformance) is fundamental. DMARC, in particular, provides visibility into your email ecosystem and allows you to enforce policies that protect your domain from unauthorized use and improve trust with mail receivers. If you're encountering DMARC verification failed errors, addressing them is key.
Engage with your HubSpot support team and Barracuda if possible, though direct contact with Barracuda can be challenging. HubSpot has resources and expertise in deliverability, and they may be able to offer insights into shared IP reputation or specific configurations relevant to Barracuda. By following best practices for email authentication and continuously monitoring your sending, you can significantly improve your email deliverability over time.
Strengthening email authentication
Properly configured authentication records are non-negotiable for good deliverability.
SPF: Ensure your SPF record includes HubSpot's sending IPs.
DKIM: Set up DKIM signing through HubSpot for your sending domain.
DMARC: Implement a DMARC policy, starting with p=none to monitor, then gradually moving to quarantine or reject.
Views from the trenches
Best practices
Implement a very gradual IP warming schedule tailored specifically for Barracuda-protected domains, recognizing their aggressive filtering.
Rigorously clean your email list by removing unengaged contacts and those with a history of bounces or inactivity.
Regularly monitor bounce messages for specific codes and patterns, especially those related to content or domain issues.
Ensure all email authentication records (SPF, DKIM, DMARC) are correctly configured and aligned with your new sending platform.
Common pitfalls
Relying solely on reported opens and clicks as engagement metrics, as Barracuda's link-following can artificially inflate these numbers.
Attempting to send large volumes to Barracuda-protected domains immediately after migrating to a new email service provider.
Not segmenting your audience and treating all recipients the same, especially when certain domains are causing high bounce rates.
Overlooking the impact of shared IP reputation when using a new email platform, as other senders can affect your deliverability.
Expert tips
If Barracuda is the primary blocker, engage with your recipients directly through alternative channels to request whitelisting.
Focus on click engagement rather than opens, especially with B2B audiences where email security appliances might skew open rates.
Actively engage with your new ESP's support team to inquire about specific Barracuda blocklist issues or shared IP reputation concerns.
Understand that a 'blocked' bounce isn't always a permanent failure; it often indicates a filtering decision that can be overcome with better sending practices.
Marketer view
Marketer from Email Geeks says they started seeing a 15% bounce rate primarily from Barracuda after moving to HubSpot, despite those contacts being highly engaged on their old platform.
October 4, 2023 - Email Geeks
Expert view
Expert from Email Geeks says that HubSpot's categorization of bounces as 'spam' for Barracuda-blocked emails is likely accurate, as the 550 permanent failure message often indicates content or domain-related filtering.
October 4, 2023 - Email Geeks
Conclusion: Navigating the post-migration landscape
Migrating email sending platforms is a complex process, and encountering high Barracuda bounce rates with HubSpot is a clear sign that your new sending profile is being viewed with suspicion. This is usually not an indictment of your list quality or your content in isolation, but rather Barracuda's reaction to a significant change in sending behavior and originating IPs.
The key to resolving these issues lies in a multi-faceted approach. Focus on a conservative IP warming strategy specifically for domains protected by aggressive filters like Barracuda. Simultaneously, ensure your email authentication (SPF, DKIM, DMARC) is perfectly aligned with HubSpot's requirements. Maintain excellent list hygiene, removing any unengaged or questionable contacts to protect your sender reputation.
By understanding Barracuda's mechanisms and proactively addressing the technical and reputational shifts caused by migration, you can successfully navigate these challenges. It requires patience, diligent monitoring, and a willingness to adapt your sending strategy to build trust with strict email security gateways.
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Amazon SES backbone) lack a positive history with Barracuda.
HubSpot's sending IPs.
