What filter uses [VI-1] when blocking email and which company is responsible?
Matthew Whittaker
Co-founder & CTO, Suped
Published 16 Apr 2025
Updated 16 Aug 2025
8 min read
Cryptic messages like [VI-1] can be frustrating for anyone managing email deliverability. These error codes often mean that your messages are being blocked by an email filter, but they provide little immediate information about the cause or the responsible entity. Understanding these technical nuances is essential for diagnosing and resolving email delivery issues, ensuring your communications reach their intended recipients.
The specific message, [VI-1], indicates that a filtering system has intercepted and blocked your email. Pinpointing precisely which filter uses this identifier and the company behind it requires a bit of detective work. Details about these internal codes are rarely publicly advertised, making the troubleshooting process more complex than it might seem.
Decoding the [VI-1] filter message
The [VI-1] code is not a standard, universally recognized email error code. Instead, it is an internal classification likely employed by a specific, proprietary email filtering system. Based on observations within the email deliverability community, this error often appears when mail is being blocked by a Mail Exchanger (MX) record owned by Synacor. Synacor is a technology company that provides various services, including email platforms, to numerous internet service providers (ISPs) and cable companies.
While the email banner might indicate ecelerity, which refers to MessageSystems (now Momentum), it's crucial to understand that Momentum is an email transfer agent (MTA). An MTA is responsible for sending and receiving email, but it does not inherently include the specific filtering logic. Instead, MTAs like Momentum can integrate various third-party spam and antivirus filters. Therefore, the ecelerity banner points to the mail transport software, not necessarily the specific content or reputation filter that generated the [VI-1] message.
There was initial speculation that the 'V' in [VI-1] might refer to Vade Secure (or Vade), a prominent email security provider. However, an analysis of bounce messages from known Vade users, such as Earthlink, shows that their block messages typically do not contain the [VI-1] identifier. This suggests that if Vade's technology is involved, it's likely part of a customized or layered filtering setup specific to Synacor's implementation for its clients, rather than a direct Vade block.
Identifying responsible entities
Identifying the exact company responsible for blocking an email with a [VI-1] code can be challenging because many large ISPs and cable companies do not manage their email infrastructure entirely in-house. Instead, they frequently outsource or white-label their email services from providers like Synacor. This means the blocking decision, while impacting the ISP's customers, often originates from the underlying service provider's systems and their specific filtering rules.
You will frequently encounter this [VI-1] block from various cable companies, including Optonline.net, Fuse.net, TDS.net, Charter.net, and RR.com domains (Roadrunner, now part of Spectrum/Charter). These companies frequently use Synacor's platform for their email services, which explains the commonality in their bounce messages. To confirm, checking their MX record history can provide a clear indication of their underlying email infrastructure. We also have more information on Optimum, Windstream, and CenturyLink filtering methods.
To investigate further, you can perform an MX lookup for the recipient's domain. This will reveal which mail servers are responsible for handling incoming mail for that particular domain. If the MX record points to a domain associated with Synacor, it strongly suggests that their filtering system is responsible for the [VI-1] block.
MX record lookup examplebash
dig MX yourdomain.com +short
The nuances of email filtering architectures
Email filtering is a multi-layered defense system, not a single monolithic block. It begins even before an email fully transmits, with connection-level filtering that checks IP reputation. Then it proceeds through header analysis, content scanning, and finally, sender authentication checks like SPF, DKIM, and DMARC. Each of these layers can contribute to a block, and the [VI-1] code likely indicates a specific rule trigger within one of these complex layers.
While Synacor operates the MX, the actual filtering decisions could be based on their proprietary algorithms, a third-party anti-spam engine they resell, or even custom rules implemented by the individual cable companies using Synacor's platform. This modularity means that even if the underlying infrastructure is the same, subtle differences in configuration can lead to varied blocking behaviors across different ISPs using the same core platform. Understanding these complexities is key to troubleshooting email deliverability issues and identifying what spam filter a company is using.
The interplay between a core platform like Synacor and individual ISP configurations highlights a common challenge in email deliverability. It is often not a simple matter of one single filter, but a combination of factors that cause messages to be blocked. For instance, sometimes blacklist email spam filters work by blocking emails from senders that have been put on a list of spammers.
Centralized filtering
Shared infrastructure: Many ISPs use a common provider, leading to similar filtering patterns.
Consistent rules: Policies are set at the provider level, affecting all their client ISPs.
Easier troubleshooting: If you identify the central system, a single solution might fix multiple blocks.
Decentralized/Custom Filtering
Varied block messages: Even with a common platform, individual ISPs can layer their own rules.
Harder to pinpoint: The specific reason for a block might be unique to a single ISP's configuration.
Custom rule sets: ISPs can subscribe to different third-party blacklists or implement internal reputation systems, as explained in our guide to public versus private blacklists.
Mitigating blocks and ensuring deliverability
To mitigate blocks, especially from filters that produce cryptic messages like [VI-1], focusing on your overall sender reputation is paramount. This includes maintaining low spam complaint rates, avoiding spam traps, and ensuring your sending IP and domain are not listed on any major email blocklists (or blacklists). Adhering to sender guidelines from major mailbox providers, such as those published by Google, is also crucial for consistent inbox placement. You can learn more about why emails go to spam and how to fix it.
Beyond reputation, robust email authentication protocols are non-negotiable. Properly configured SPF, DKIM, and DMARC records signal to receiving servers that your emails are legitimate and not spoofed. A common reason for filtering decisions, including those from systems like the one behind [VI-1], is a failure in these authentication checks. Additionally, complying with regulations such as the CAN-SPAM Act further ensures legal compliance and positive sender standing.
Regularly monitoring your email deliverability and any bounce messages is vital. While [VI-1] might be specific, generic bounce messages can also signal issues that need attention. Pay close attention to common pitfalls that lead to blocks, such as high complaint rates or sending to unengaged recipients. A comprehensive email deliverability tester can help identify potential issues before they impact your sending.
The [VI-1] filter message points to a sophisticated email blocking system, most likely managed by a company like Synacor, which provides email infrastructure for numerous cable and internet service providers. While the MTA might be Momentum (ecelerity), the specific filtering logic is integrated into their broader system and can be quite complex. These integrated systems often combine various techniques, including reputation checks and content analysis, to identify and block unwanted mail. This is often part of ISP practices for handling suspicious email.
Navigating these nuances requires a proactive approach to email deliverability. By focusing on strong sender reputation, proper authentication, and consistent monitoring of your email campaigns, you can significantly improve your chances of reaching the inbox, even when encountering less common bounce messages. Ultimately, a holistic strategy is key to long-term success in the ever-evolving landscape of email security and filtering.
Views from the trenches
Best practices
Ensure robust email authentication (SPF, DKIM, DMARC) is correctly configured for your sending domains.
Regularly clean your email lists to remove inactive or invalid addresses, preventing bounces and spam trap hits.
Monitor your domain and IP reputation using tools and postmaster feedback loops from major mailbox providers.
Maintain consistent sending volumes and avoid sudden spikes that could trigger suspicious behavior flags.
Provide clear and easy unsubscribe options in all your marketing emails to reduce complaint rates.
Common pitfalls
Failing to monitor bounce messages and error codes, leading to prolonged deliverability issues.
Relying solely on a generic MTA without understanding the underlying filtering mechanisms.
Neglecting to update your sending infrastructure or scripts when ISPs change their MX records or filtering providers.
Sending to unengaged lists, which increases spam complaints and negatively impacts sender reputation.
Not segmenting email campaigns, leading to irrelevant content being sent to recipients, increasing filtering risk.
Expert tips
Implement DMARC with a monitoring policy (p=none) to gain visibility into your email authentication failures.
Use a dedicated IP address for high-volume sending to build and maintain a consistent sender reputation.
Warm up new IP addresses gradually to establish a positive sending history with mailbox providers.
Engage with deliverability communities and forums to stay updated on ISP changes and filtering trends.
Test your email content and rendering across various email clients and devices before sending large campaigns.
Expert view
Expert from Email Geeks says: Understanding that filter responses are often classified manually based on MX records and internal client data is crucial for diagnosing widespread deliverability issues.
2022-01-13 - Email Geeks
Marketer view
Marketer from Email Geeks says: Always double-check MX changes for major ISPs, as shifts in their infrastructure, like those seen with Optonline.net and Charter, can impact sending scripts and deliverability.
Synacor. Synacor is a technology company that provides various services, including email platforms, to numerous internet service providers (ISPs) and cable companies.
Momentum), it's crucial to understand that Momentum is an email transfer agent (MTA). An MTA is responsible for sending and receiving email, but it does not inherently include the specific filtering logic. Instead, MTAs like Momentum can integrate various third-party spam and antivirus filters. Therefore, the ecelerity banner points to the mail transport software, not necessarily the specific content or reputation filter that generated the [VI-1] message.
Vade Secure (or Vade), a prominent email security provider. However, an analysis of bounce messages from known Vade users, such as 
Earthlink, shows that their block messages typically do not contain the [VI-1] identifier. This suggests that if Vade's technology is involved, it's likely part of a customized or layered filtering setup specific to Synacor's implementation for its clients, rather than a direct Vade block.
Optonline.net, 
Fuse.net, 
TDS.net, 
Charter.net, and 
RR.com domains (Roadrunner, now part of Spectrum/Charter). These companies frequently use Synacor's platform for their email services, which explains the commonality in their bounce messages. To confirm, checking their MX record history can provide a clear indication of their underlying email infrastructure. We also have more information on Optimum, Windstream, and CenturyLink filtering methods.
