How to resolve email blocking issues with Optimum/Optonline/Altice/Synchronoss?
Michael Ko
Co-founder & CEO, Suped
Published 10 Aug 2025
Updated 19 Aug 2025
6 min read
Email blocking issues with internet service providers (ISPs) like Optimum, Optonline, Altice, and Synchronoss can be incredibly frustrating. These providers handle a significant volume of email traffic, and any block can severely impact your communication, whether for marketing, transactional, or personal messages.
The challenge often lies in identifying the root cause of the block, as these providers typically employ complex filtering systems that can be opaque. From IP blacklists (or blocklists) to content filters, a range of factors can lead to your emails not reaching the intended recipients.
This guide provides practical steps and insights to help you diagnose and resolve email blocking issues specific to the Optimum (including Optonline and Altice) and Synchronoss networks, offering strategies to improve your email deliverability and ensure your messages land in the inbox.
Diagnosing the blocking issue
Before diving into specific troubleshooting, it is essential to understand why Optimum/Optonline/Altice/Synchronoss might be blocking your emails. Their systems are designed to protect users from spam and malicious content, meaning legitimate emails can sometimes be caught in the crossfire.
Common reasons include your sending IP address or domain being listed on an external blocklist (also called a blacklist), poor sender reputation, or content that triggers their spam filters. Historically, mail services for these domains were provided by Synchronoss, which later became part of Altice USA (Optimum/Optonline), so understanding this lineage helps explain their shared infrastructure and policies. You can find more details on their spam management policies directly from Optimum's official page.
The first step is to confirm that you are indeed being blocked. Check your bounce messages, as they often contain specific error codes or messages indicating the reason for rejection, such as 554 5.7.1 or references to blocked by xxx.rbl. This information is crucial for pinpointing the exact issue, whether it is an IP block (blacklist) or a content-related block.
Technical solutions and best practices
Once you have identified the nature of the block, you can take targeted actions. If your IP address is on a public blocklist (or blacklist), you need to request delisting. Most reputable blocklists provide a delisting process on their websites, which usually involves a form submission and a review of your sending practices. For a comprehensive overview, see our guide to email blocklists.
Improving your sender reputation is a continuous effort. Ensure your email authentication records, such as SPF, DKIM, and DMARC, are correctly configured. These records help mail servers verify that your emails are legitimate and reduce the likelihood of them being marked as spam. A properly configured DMARC record is particularly important.
If content is the issue, review your email messages for common spam triggers, such as excessive use of caps, exclamation marks, suspicious links, or certain keywords. Test your emails with an email deliverability tester to identify potential content-related issues before sending.
Proactive monitoring and postmaster contact
Proactive measures are always better than reactive firefighting. Regularly monitor your sender reputation and blocklist status. Staying informed about your email performance can help you identify and address potential issues before they escalate into major blocking problems.
For specific issues, you might need to contact the postmaster team for Optimum, Optonline, Altice, or Synchronoss. While direct contact can be challenging, providing them with detailed information, including bounce messages and your sending IP/domain, can expedite the process. However, as noted in various forums, direct engagement with their postmaster teams can be notoriously difficult and often yield limited results. Spamresource.com often publishes updates on these providers, which can be helpful.
Consider implementing a feedback loop (FBL) with major ISPs if available. FBLs provide you with reports when users mark your emails as spam, allowing you to remove those users from your mailing list and improve your sending practices. This helps maintain a cleaner list and reduces spam complaints.
Mitigating and preventing blocks
While resolving issues with Optimum/Optonline/Altice/Synchronoss can be tricky, there are specific steps you can take to mitigate the impact and prevent future blocks. Focus on the fundamentals of email deliverability and maintain a high level of compliance.
List hygiene: Regularly clean your email lists by removing inactive subscribers, bounced addresses, and spam trap hits. Sending to a clean list reduces complaint rates and improves your sender reputation. For more on this, consider our guide on resolving Optonline IP blacklisting.
Content quality: Craft engaging and relevant content. Avoid anything that might be perceived as spammy or misleading. Personalization and segmentation can help improve engagement.
Subscription management: Provide clear and easy unsubscribe options. A single-click unsubscribe header (List-Unsubscribe header) is highly recommended and often a requirement for major inbox providers.
For persistent issues, especially related to password reset emails or critical transactional messages, consider exploring alternative delivery mechanisms for the most sensitive communications, or advise your users to use alternative email addresses from providers like Gmail or Outlook. This can be a short-term workaround while you address the underlying deliverability problems.
Key takeaways for reliable delivery
Addressing email blocking issues with providers like Optimum, Optonline, Altice, and Synchronoss requires a multi-faceted approach. It involves understanding their policies, diligent monitoring, strong email authentication, and consistent adherence to email best practices.
While direct resolutions can be challenging, maintaining a robust email program built on good sender reputation, proper authentication, and engaging content will always be your best defense against deliverability challenges. For more insights, learn about why emails fail.
Views from the trenches
Best practices
Maintain strong email authentication (SPF, DKIM, DMARC) for all sending domains.
Segment your email lists and personalize content to reduce spam complaints.
Regularly monitor your sending IP and domain for blacklisting (blocklisting) on major DNSBLs.
Common pitfalls
Ignoring bounce messages and delivery failure reports.
Sending emails to old or unengaged addresses, which can lead to spam traps.
Not having a designated contact or process for ISP postmaster outreach.
Expert tips
Advise users experiencing persistent blocking to use a major webmail provider for critical communications.
Focus on overall sending reputation rather than fixating on a small percentage of unresolvable blocks.
Keep an eye on industry forums and resources for updates on ISP filtering changes.
Expert view
Expert from Email Geeks says they found that Optimum/Optonline/Altice/Synchronoss shows absolutely no interest in resolving email blocking issues, even for legitimate transactional senders.
2022-12-15 - Email Geeks
Expert view
Expert from Email Geeks says they spent two months calling Optimum support multiple times per week, with very little meaningful feedback, until the issue seemingly resolved itself.
Gmail or 
Outlook. This can be a short-term workaround while you address the underlying deliverability problems.
