Why are Optonline.net emails bouncing but sometimes still delivering?
Michael Ko
Co-founder & CEO, Suped
Published 8 May 2025
Updated 16 Aug 2025
6 min read
It can be incredibly frustrating when you send important emails, only to find some are bouncing, yet others are successfully reaching recipients at the same domain. This inconsistent behavior is particularly prevalent with internet service providers (ISPs) like Optonline.net. You might encounter bounce messages like "554 maximum message size exceeded" even for plain text emails, or "550 5.7.1 Unacceptable", while other messages sail through. Understanding why this happens requires a deeper look into how email systems, especially those run by major ISPs, handle incoming mail.
The complexities of Optonline.net's email filtering
Optonline.net, operated by Optimum (Altice), is known for its aggressive email filtering policies. Their systems are designed to protect users from spam, but this often leads to legitimate emails being incorrectly identified as unwanted. This over-blocking can result in many false spam bounces, preventing valid communications from reaching their intended inboxes. It's a challenging situation for senders, as the rules for acceptance can seem opaque and inconsistent.
A significant factor contributing to this behavior is Optonline.net's reliance on both public and internal blocklists (sometimes called blacklists). While your sending IP address or domain might not appear on commonly checked public blocklists, Optimum (Optonline) maintains its own proprietary internal lists. If your sending practices, even seemingly benign ones, trigger their internal thresholds, your emails can be delayed or rejected, leading to bounces. Digitalfaq.com discusses how they often over-block and create false spam bounces.
It's common for senders to report that Optimum Online will not accept email transactions from IP addresses that appear on any of the block lists it uses. This indicates a very strict stance on sender reputation. The challenge is that these internal lists are not transparent, making it difficult to pinpoint the exact reason for a specific block or bounce.
The fluctuating deliverability, where some emails get through and others bounce, often points to dynamic filtering based on real-time reputation scores or content analysis. This means that a slight change in email volume, content, or recipient engagement can tip the balance, leading to inconsistent outcomes.
Decoding Optonline.net bounce messages
Bounce messages provide critical clues, even if they sometimes seem contradictory. The "554 maximum message size exceeded" error for a small, text-based email is a classic example of Optonline's filtering at play. This isn't usually about the actual size of the email, but rather a generic rejection message that the system uses when it doesn't want to accept mail for other, often unstated, reasons. It can be a content-based filter or a reputation trigger.
Another common bounce error you might encounter is "550 5.7.1 Unacceptable message content" or a similar variation. This generally indicates that Optonline's spam filters have identified something within your email's content, headers, or links that they deem suspicious. This could be anything from certain keywords, specific formatting, or even the inclusion of particular URLs.
Understanding these bounce codes is the first step in diagnosing the problem. While Optonline.net doesn't provide a public postmaster site with detailed explanations for every code, the context of your sending practices often helps to infer the underlying cause. If you're seeing a spike in these bounces, it suggests a recent change in your sending behavior or a heightened sensitivity in their filtering.
Bounce type
Common meaning for Optonline.net
Actionable insights
"554 maximum message size exceeded"
Often a generic rejection code, not necessarily size-related. Indicates a deeper content or reputation issue.
Content or sender reputation flagged as spam by Optonline's filters.
Refine email content, warm up IP/domain if new, ensure strong authentication.
"Mailbox unavailable" or "No such user"
The recipient's email address does not exist or is no longer active.
Remove invalid addresses from your mailing list immediately.
"Connection timed out" or "Service unavailable"
Temporary issues with Optonline's servers, or high volume sending causing throttling.
Reduce sending rate, monitor server status, check for connection limits.
Why partial deliveries happen
The perplexing aspect of Optonline.net deliverability is the intermittent success. You're getting bounces, but some emails are still landing in inboxes. This usually comes down to the dynamic nature of reputation and filtering. Optonline's mail servers evaluate each incoming connection and message based on a complex algorithm that considers various factors.
Sender reputation plays a huge role. If your sender reputation is generally good, but you hit a specific trigger (e.g., a sudden increase in volume, a single suspicious keyword, or a minor content change), some of your emails might be momentarily flagged, leading to a bounce for those specific messages, while others from the same batch that didn't trip the wire might go through. Optonline.net, much like Gmail or Yahoo, uses sophisticated systems that learn and adapt.
Another factor is connection limits or rate limiting. ISPs often impose limits on how many emails an IP address can send per hour or per connection. If you exceed these limits, Optonline might temporarily reject some emails, leading to a bounce. This helps them manage server load and prevent abuse. Our article on what causes connection limit issues with Optonline.net recipients provides more details on this.
Important: Partial delivery factors
Sender reputation: Overall trust score of your sending IP and domain.
Content filtering: Certain words, links, or formatting that trigger spam algorithms.
Recipient-specific settings: Individual user blocklists or spam preferences.
Connection limits: Throttling based on the volume of emails sent within a time frame.
Strategies for improving Optonline.net deliverability
Navigating Optonline.net's stringent filters requires a proactive approach focused on maintaining a pristine sender reputation and adhering to email best practices. This is not about a quick fix, but a continuous effort to demonstrate legitimacy and value to their systems.
One of the most crucial steps is to ensure your email authentication, including SPF, DKIM, and DMARC, is correctly configured. These protocols help Optonline's servers verify that your emails are legitimate and haven't been spoofed, significantly impacting your trustworthiness. Our simple guide to DMARC, SPF, and DKIM can help you set these up properly.
Before: common Optonline.net problems
High bounce rates: Seeing "554 maximum message size exceeded" or "550 5.7.1 Unacceptable" for valid emails.
Inconsistent delivery: Some emails get through, others bounce, making campaigns unreliable.
Unexplained blocks: Emails are rejected without clear reasons, even if not on public blocklists.
Throttling: Experience delays or temporary rejections due to sending too many emails quickly.
Low sender reputation: Your domain or IP is flagged by Optonline's internal systems.
After: effective deliverability solutions
Robust authentication: Implement strong SPF, DKIM, and DMARC records.
List hygiene: Regularly clean your email lists to remove invalid Optonline.net addresses.
Content optimization: Avoid spammy keywords, excessive images, or suspicious links.
Rate limiting: Adjust sending volumes to align with Optonline's implicit thresholds.
Engagement focus: Encourage opens, clicks, and replies from Optonline.net recipients.
If you've exhausted all troubleshooting steps and still face persistent issues, reaching out to the Optonline.net postmaster team directly can sometimes provide insights, though responses may vary. Our guide on how to successfully contact Optonline postmaster can assist with this. Remember, persistence and adherence to best practices are key to improving your email deliverability with challenging ISPs.
Views from the trenches
Best practices
Regularly audit your email content for potential spam triggers, even for innocuous text or image ratios.
Implement and monitor SPF, DKIM, and DMARC to build and maintain strong sender authentication.
Segment your Optonline.net recipients and send in smaller, more consistent batches to avoid hitting rate limits.
Proactively remove unengaged Optonline.net subscribers to improve overall list quality and engagement metrics.
Sending to stale or unverified Optonline.net email addresses, which leads to hard bounces and spam trap hits.
Assuming that a clean record on public blacklists guarantees delivery to Optonline.net, ignoring their internal blocklists.
Sending high volumes of emails in short bursts, triggering Optonline.net's rate limiting or throttling mechanisms.
Expert tips
Validate your email lists regularly before sending, especially for Optonline.net addresses, to minimize bounces.
Pay close attention to engagement metrics for your Optonline.net audience; low engagement can impact deliverability.
If you see a sudden spike in Optonline.net bounces, review recent changes to your email content or sending patterns.
Use a dedicated IP address for high-volume sending to Optonline.net to better control your sender reputation.
Marketer view
Marketer from Email Geeks says: I experienced a sudden increase in emails bouncing to Optonline.net domains, specifically with a '554 maximum message size exceeded' error.
2022-03-16 - Email Geeks
Marketer view
Marketer from Email Geeks says: These bounces were occurring for standard mass emails, which were not large and contained only text and a few small images, ruling out actual size as the cause.
2022-03-16 - Email Geeks
Final thoughts on Optonline.net deliverability
Inconsistent email delivery to Optonline.net recipients, characterized by simultaneous bounces and successful deliveries, is a common frustration for senders. It stems from their strict, often opaque, filtering mechanisms, which leverage both public and internal blocklists, alongside dynamic content and reputation-based evaluations. By diligently managing your sender reputation, ensuring robust email authentication, and maintaining clean email lists, you can significantly improve your chances of consistently reaching Optonline.net inboxes.
Optonline.net. You might encounter bounce messages like "554 maximum message size exceeded" even for plain text emails, or "550 5.7.1 Unacceptable", while other messages sail through. Understanding why this happens requires a deeper look into how email systems, especially those run by major ISPs, handle incoming mail.
Gmail or 
Yahoo, uses sophisticated systems that learn and adapt.
Implement strong SPF, DKIM, and DMARC records.
