Why are my emails blocked by Barracuda even when not listed on blocklists?
Michael Ko
Co-founder & CEO, Suped
Published 18 May 2025
Updated 17 Aug 2025
6 min read
It can be incredibly frustrating to find your emails blocked by Barracuda when a check of common blacklists or blocklists shows your IP and domain are clean. You might assume if you're not on a public blacklist (or blocklist), your emails should flow freely, but Barracuda's filtering goes deeper than just those widely known lists.
Barracuda Networks, a prominent email security provider, uses a sophisticated, multi-layered approach to identify and block unwanted emails. This means that even if your sending infrastructure (IP address, domain) isn't listed on major public blocklists like Spamhaus or SpamCop, other factors can still trigger a block by a Barracuda appliance or service.
Understanding these underlying mechanisms is crucial for diagnosing and resolving why your legitimate emails might be getting blocked. It often comes down to reputation, content, or specific policies configured by the recipient's Barracuda system.
Barracuda's filtering system operates on several levels, extending beyond simple IP or domain blacklisting (or blocklisting). They maintain their own proprietary blocklist, the Barracuda Reputation Block List (BRBL), which is fed by their extensive network of deployed security devices and spam traps. This internal list might flag your IP or domain based on criteria that aren't public.
Beyond the BRBL, Barracuda employs advanced content analysis. This means they scrutinize the actual content of your emails for characteristics often associated with spam, such as suspicious links, unusual formatting, certain keywords, or hidden text. If the content analysis flags your message, it can be blocked regardless of your sender's reputation on external lists.
Individual organizations using Barracuda products can also set up custom policies, allowlists (whitelists), and blocklists (blacklists) that override general reputation scores. This means a specific recipient or their IT department might have manually blocked your sending domain, email address, or even certain content patterns, leading to a block that's localized to them.
Here's a breakdown of how Barracuda evaluates incoming email:
Filtering Mechanism
Description
Impact on Deliverability
Barracuda Reputation Block List (BRBL)
Barracuda's proprietary blocklist of IPs and domains known for sending spam.
Immediate blocking if your IP/domain is on this list, even if not on public ones.
Content analysis
Scans email body, subject, and links for spam characteristics, including hidden content.
High spam score can lead to blocking or quarantining, regardless of sender reputation.
Recipient-specific policies
Custom allowlists, blocklists, and content rules set by individual Barracuda users or administrators.
Can block messages from senders who would otherwise pass general filters.
Sender Policy Framework (SPF), DKIM, DMARC
Checks email authentication records to verify sender legitimacy.
Failure to align or missing records can result in emails being flagged or blocked.
Beyond the blocklist: common blocking triggers
Even without a public blocklist entry, several factors can trigger Barracuda's filters. One common cause is a poor sender reputation (even an unlisted IP address) built up through spam complaints or hitting spam traps, even if those incidents don't land you on a major public blacklist. Barracuda might have its own internal data that indicates problematic sending behavior from your IP or domain.
Another frequent culprit is misconfigured or missing email authentication records. While SPF, DKIM, and DMARC help verify your sending legitimacy, errors in these records can cause Barracuda to view your emails with suspicion. For example, if your SPF record is too long, contains too many lookups, or isn't properly aligned, Barracuda might treat emails as unauthenticated or suspicious.
Sometimes, the issue isn't with your sending, but with the recipient's domain. If a domain is no longer active or has misconfigured DNS, Barracuda might block emails to it. For instance, if an email is sent to an email address at a domain that previously existed but whose web presence has vanished, Barracuda may reject it with a permanent failure message like '550 permanent failure for one or more recipients: blocked'. This can happen even if the email domain still technically has MX records.
Here are common causes of Barracuda blocks and how they manifest:
Typical problems
Sender reputation: Despite not being on public blocklists, internal Barracuda scores may be low due to prior spam complaints, sending to spam traps, or inconsistent sending volumes.
Email content: Spammy keywords, suspicious links, excessive images, or poor HTML formatting can trigger content filters.
Spam folder delivery: Email is delivered but routed to the spam or junk folder, which Barracuda also manages.
Diagnosing unexpected Barracuda blocks
When facing Barracuda blocks, the first step is to carefully examine the bounce message. Barracuda's error messages often provide specific clues, such as "550 permanent failure for one or more recipients (email@domain.com:blocked)" or references to their ESS (Email Security Service) or BRBL.
Next, use the Barracuda Central IP/Domain Lookup tool. While this tool primarily shows if you're on the public-facing BRBL, it's a good starting point. If you're clear there, the issue likely stems from content, authentication, or recipient-specific rules.
I always recommend performing an inbox placement test with a range of email providers, including those protected by Barracuda. This will reveal if the problem is widespread or isolated to specific Barracuda-protected domains. If test emails successfully inbox with Barracuda seeds, it points towards highly localized recipient-side policies.
Deciphering a Barracuda bounce message
Here's a typical example of a Barracuda bounce message, and what key elements to look for:
Example Barracuda bounce message
Google tried to deliver your message, but it was rejected by the server for the recipient domain example.com by d241253a.ess.barracudanetworks.com. [209.222.82.252]. The error that the other server returned was: 550 permanent failure for one or more recipients (recipient@example.com:blocked)
Recipient domain: Confirm the domain (example.com) is correct and active. Sometimes, blocks occur because the recipient's domain itself has issues, not necessarily your sending practices.
Barracuda host: The Barracuda server (d241253a.ess.barracudanetworks.com) indicates it's a Barracuda filter. The IP address in brackets is the Barracuda server IP, not necessarily your sending IP.
Error code: '550 permanent failure' means the email was definitively rejected and will not be retried. The '(recipient@example.com:blocked)' part indicates a specific block for that recipient.
Strategies for resolution and prevention
To resolve Barracuda blocks when you're not on a public blacklist, direct communication with the recipient's IT team or their email administrator is often the most effective route. They can check their Barracuda logs for the specific reason for the block and manually allowlist your sending IP or domain. This is especially true if the block is isolated to a few recipients, as it points to specific internal policies.
For broader issues, focus on enhancing your sender reputation. This includes maintaining a clean email list, avoiding purchased lists (which are prone to spam traps), and promptly removing inactive or bouncing addresses. Implement a double opt-in process for new subscribers to ensure engagement and minimize spam complaints. Pay attention to engagement metrics: low open rates or high unsubscribe rates can negatively impact your reputation with filters like Barracuda.
Finally, ensure your email authentication (SPF, DKIM, DMARC) is perfectly configured. These records demonstrate your legitimacy as a sender. Use tools to verify that your records are valid and aligned. If you are using a shared IP, this can sometimes lead to issues, but there are solutions for shared IP blocklists as well. Barracuda tends to be very strict on these authentication checks, so any misconfiguration can result in an unexpected block.
Tips for improving Barracuda deliverability
Monitor bounce rates: High bounce rates, especially hard bounces, can signal problems with your recipient list or sending practices. Identify and remove invalid addresses.
Review email content: Avoid spammy phrases, excessive capitalization, and image-only emails. Ensure a good text-to-image ratio and clear calls to action. Use a plain text alternative.
Sender Policy Framework (SPF): Ensure all legitimate sending IPs and services (e.g., Trend Micro, Microsoft Outlook) are included, and no outdated or incorrect entries are present.
DomainKeys Identified Mail (DKIM): Confirm your DKIM signature is valid and correctly applied to your outgoing emails.
Maintain meticulous list hygiene, regularly removing unengaged or invalid email addresses.
Implement DMARC with monitoring enabled to gain visibility into authentication failures.
Consistently test your email deliverability to various providers, including Barracuda-protected domains.
Ensure all email authentication records (SPF, DKIM, DMARC) are accurately configured and up-to-date.
Common pitfalls
Relying solely on public blacklist checks without investigating internal Barracuda reputation scores.
Ignoring subtle bounce messages that indicate recipient-specific blocks rather than general blocklist issues.
Failing to communicate directly with recipient IT teams for targeted delisting requests.
Using generic or low-quality email content that triggers Barracuda's advanced content filters.
Expert tips
If the block is specific to a recipient, try sending a plain text email with minimal content from a different, trusted source to verify if the issue is content-related or a hard block.
Review your sending logs for multiple failure messages from Barracuda for different recipients, which would suggest a broader issue rather than an isolated one.
Consider contacting Barracuda support directly with the bounce message details, even if your domain/IP is not listed on Barracuda Central.
Ensure your DNS records, particularly MX records, are clean and consistent across all sending infrastructure to avoid confusion for email security gateways.
Expert view
Expert from Email Geeks says that the issue might be a block imposed by a specific recipient.
2024-12-06 - Email Geeks
Marketer view
Marketer from Email Geeks suggests checking if the block occurred after Barracuda scanned links, indicating a potential content-related issue.
2024-12-06 - Email Geeks
Navigating Barracuda email blocks
Email deliverability to Barracuda-protected domains requires a holistic approach that extends beyond merely checking public blacklists (or blocklists). Barracuda's robust filtering considers a combination of factors, including its internal reputation lists, email content analysis, and custom recipient policies.
By diligently managing your sender reputation, ensuring flawless email authentication, crafting clean and compliant email content, and proactively engaging with recipients when blocks occur, you can significantly improve your chances of consistently reaching the inbox. Remember, proactive monitoring and quick diagnosis are key to maintaining strong email deliverability.
Spamhaus
0Spam
Cisco
NoSolicitado
URIBL
abuse.ro
ALPHANET
Anonmails
Ascams
BLOCKEDSERVERS
Calivent Networks
EFnet
JustSpam
Kempt.net
NordSpam
RV-SOFT Technology
Scientific Spam
Spamikaze
SpamRATS
SPFBL
Suomispam
System 5 Hosting
Team Cymru
Validity
www.blocklist.de Fail2Ban-Reporting Service
ZapBL
2stepback.dk
Fayntic Services
ORB UK
technoirc.org
TechTheft
Ensure all legitimate sending IPs and services (e.g., Trend Micro, Microsoft Outlook) are included, and no outdated or incorrect entries are present.
