What causes high Barracuda bounce rates after migrating to HubSpot and how to resolve them?

Key findings

• Sudden increase: A high percentage of bounces from Barracuda immediately after migrating to a new ESP like HubSpot, even for previously engaged contacts, suggests a change in sender identity that Barracuda's filters are reacting to.
• Bounce message context: A 550 permanent failure for one or more recipients (:blocked) message from Barracuda typically indicates content or domain-related blocking, or that the sender's IP or domain has hit a certain score threshold, leading to rejection. Despite being a 'permanent failure,' a different message could potentially deliver later.
• Shared IP challenges: HubSpot often uses shared IP addresses, which means your email sending reputation is tied to the actions of other users on the same IP. If other senders on that shared IP have poor practices, it can negatively affect your deliverability, leading to blocklisting by services like Barracuda.
• Engagement metric accuracy: Barracuda's filters are known to pre-scan emails and follow links, which can inflate open and click metrics on your old platform. This means that some reported engagement might have been Barracuda's own checks, not genuine recipient interaction.
• Domain vs. IP reputation: Even if your sending domain remains the same, the change in IP address (from the old ESP to HubSpot's shared IPs) can be a significant trigger for Barracuda's filters, leading to increased scrutiny and potential blocking.

Key considerations

• Investigate bounce specifics: Obtain the exact bounce messages from HubSpot for each Barracuda bounce to pinpoint the reason. The SMTP Field Manual can offer insights into Barracuda's specific error codes.
• Gradual IP warming: Even with a list of highly engaged contacts, a rapid shift to a new ESP's IP can appear suspicious. Implement a very slow and controlled IP warming process on HubSpot, especially for B2B domains protected by Barracuda. Consider segmenting your list by domain and warming up those domains with Barracuda protection even more slowly.
• Engage HubSpot support: Liaise with HubSpot's support team to understand their shared IP reputation, any known issues with Barracuda, and what steps they can take to mitigate the problem.
• Recipient outreach: Barracuda's support often directs senders to have recipients contact their IT team. While impractical for a large list, this highlights that the block might be client-side. Identifying the key recipient domains affected by Barracuda bounces can help target specific account-level outreach, if feasible.
• Clean your list: Even highly engaged lists can contain stale addresses. Regularly verifying your email list helps reduce hard bounces and improves overall sender reputation. Barracuda may also block if it detects suspicious patterns, even if the address was once valid on an old ESP. For more on this, see how to troubleshoot high soft bounce rates.

Key opinions

• ESP support is key: Many marketers suggest that the first step should always be to engage with the new ESP's support team, in this case, HubSpot, to understand their deliverability posture and any specific recommendations for Barracuda.
• Shared IP suspicion: A common opinion is that shared IP addresses on new platforms can be the culprit due to the actions of other senders on the same IP, which might have previously triggered Barracuda's blacklists or blocklists.
• Content-based blocking: Marketers frequently point to content-based rules (e.g., specific links, formatting, or keywords) as a reason for Barracuda blocks, especially when migrating, as filtering criteria can change.
• Gradual migration: Phased migrations, where highly engaged segments are moved first and then progressively less engaged segments, are generally considered best practice, but even this needs careful execution.

Key considerations

• Monitor bounce messages: Always capture and analyze the exact bounce message, as it provides critical clues for diagnosing the issue, even if the ESP categorizes it broadly as 'spam'.
• Verify engagement metrics: Be aware that Barracuda's link following can distort open and click metrics, meaning past 'engagement' might not be a true reflection of recipient interaction, especially for B2B lists. This also highlights the importance of keeping your mailing list fresh and regularly cleaned.
• Segment by domain: When facing widespread Barracuda bounces, identify the specific domains affected and consider creating segments to apply slower warming strategies or different content approaches for these high-friction recipients.
• Temporary suppression: Temporarily suppressing problematic Barracuda contacts while continuing a controlled warmup to other domains can prevent further damage to sender reputation. This is especially true if you are dealing with deliverability drops after migrating.

Key opinions

• Barracuda's filtering behavior: Experts indicate that Barracuda often heavily filters email when it detects a sender is using a new IP address or originating from a different platform than before, even for established domains. This is part of their robust security protocol.
• Fake engagement metrics: It's a common expert opinion that Barracuda is a grandparent of systems that follows all links, meaning reported opens and clicks from your previous platform might include interactions from Barracuda's scanners rather than actual human engagement.
• B2B domain focus: Many Barracuda instances are in front of B2B domains, where employee turnover can lead to decommissioned mailboxes. Emails sent to these addresses will bounce, regardless of prior engagement.
• Shared domain blocking: Experts suggest looking into whether HubSpot has any shared domains that are already blocklisted by Barracuda, as this could impact all senders using that shared infrastructure.

Key considerations

• Rethink engagement: If Barracuda's pre-scanning created 'fake' engagement, then the list of 'highly engaged' users might not be as clean as it appears. This highlights the need for a more robust list cleaning strategy.
• Slow down warmup for Barracuda: Since Barracuda is sensitive to new sending patterns, experts advise warming up domains protected by Barracuda at an even slower pace than other segments during an ESP migration. Learn more about warming up domains in our guide on sudden increases in bounce rates.
• Limited direct contact: Unlike some blocklists, Barracuda does not provide a direct way for senders to request removal. Resolution often depends on the recipient's IT team escalating the issue or improving your sending reputation over time.
• B2B deliverability challenges: When facing blocks from B2B domains, it's recommended to look at the specific bounce codes received and identify if the same message is returned by multiple domains, which can indicate a broader issue.

Key findings

• Understanding 550 errors: Documentation for the 550 permanent failure bounce message from Barracuda indicates the block is typically due to content or domain-related reasons. It also notes that it's not always a truly 'permanent' failure, as a different message might deliver later.
• Sender reputation impact: Official guidelines emphasize that poor domain reputation, often influenced by frequent spam complaints or high bounce rates, significantly reduces the likelihood of emails reaching the inbox.
• IP warm-up necessity: Documentation on email best practices consistently highlights the importance of warming up new IP addresses or domains. Sending large volumes from a new source too quickly can trigger spam filters, regardless of list quality.
• Bounce rate thresholds: Industry recommendations, such as those from Valimail, suggest keeping bounce and complaint rates extremely low (e.g., below 0.1%) to maintain good sender health. Exceeding these thresholds can lead to severe deliverability problems.

Key considerations

• Diagnostic steps: When facing blocks from B2B domains, it's advised to thoroughly examine the bounce codes received and identify if the same bounce message is appearing across multiple domains, which can help diagnose the underlying issue.
• List hygiene: Documentation stresses that a poor quality email list with incorrect or outdated addresses is a primary cause of high bounce rates. Regular cleaning and verification of email lists are fundamental for preventing deliverability issues.
• Domain authentication: Properly configured domain authentication (SPF, DKIM, DMARC) is consistently cited as a critical factor for improving deliverability and preventing emails from being flagged as spam. It verifies your sending legitimacy to receiving servers.