Why are Optonline.net emails bouncing but sometimes still delivering?

Key findings

• Unexpected bounces: Optonline.net is returning 554 maximum message size exceeded bounces, even for emails that are not large or do not contain attachments.
• Widespread impact: The issue affects a broad range of senders, IPs, and campaigns, indicating it's not isolated to a single sending setup.
• Partial delivery: Despite bounce reports, some emails to Optonline.net addresses are still successfully reaching inboxes.
• Sudden spike: The occurrence of these specific bounces has recently spiked from previously negligible levels.
• Content type: Affected emails are typically text-based with only a few small images, challenging the maximum message size exceeded error.

Key considerations

• Email size verification: Thoroughly check the actual byte size of your emails, including all hidden elements or encoding, as these can unexpectedly inflate the total message size.
• Bounce log analysis: Monitor bounce logs for consistency of the 554 error and any other accompanying codes that might provide further insight.
• ISP-specific filtering: Optonline.net may use aggressive or dynamic filtering rules that generic bounce messages might not fully explain. It's common for some ISPs to over-block at times.
• Segmented sending: Consider segmenting your email traffic to Optonline.net recipients to isolate if specific campaigns, content types, or sending IPs are more prone to these bounces. This can be part of a broader troubleshooting approach.

Key opinions

• Unexplained spikes: Many marketers report a sudden and unexplained increase in bounces specifically to Optonline.net, diverging from previous sending patterns.
• Misleading error codes: The 554 maximum message size exceeded error often appears for emails known to be small, leading to confusion about the actual cause.
• Simultaneous delivery: A common observation is that while some emails bounce, others (sometimes even within the same send) successfully reach Optonline.net recipients.
• Content is simple: Marketers affirm that the affected emails are typically text-based, often with minimal images, ruling out obvious size issues.
• Broad impact: This issue is reported across various sending clients, IP addresses, and email campaigns, indicating a system-wide behavior from Optonline.net rather than a sender-specific problem. Similar patterns have been seen with other ISPs.

Key considerations

• Hidden size factors: What appears small on the surface might have hidden elements (e.g., complex HTML, excessive metadata, unusual character encoding) that inflate the actual message size as perceived by the receiving server.
• Dynamic filtering thresholds: Optonline.net may have dynamic filtering that adjusts based on real-time traffic, sender reputation, or even internal server load, leading to inconsistent enforcement of size limits.
• Soft vs. hard bounces: The intermittent nature of the bounces suggests they might sometimes behave like soft bounces, even if the error code implies a permanent failure.
• Monitoring: Consistent monitoring of bounce rates and specific error messages for Optonline.net is crucial to identify patterns and determine if the issue resolves itself or requires intervention.

Key opinions

• Generic error masking: Experts believe the 554 error might be a catch-all for other policy violations or spam detections that Optonline.net doesn't explicitly detail.
• Dynamic filtering: The intermittent successful deliveries suggest that Optonline.net's filtering isn't static but possibly adapts based on real-time traffic, sender volume, or overall network load.
• Hidden content: Advise a deep dive into email content for hidden or unexpected elements (e.g., obscure tracking pixels, overly complex HTML, or unusual encoding) that could inflate the message size.
• Reputation impacts: Even for legitimate senders, a slight dip in reputation or increased volume could trigger more aggressive filtering, leading to bounces that appear unrelated to content.
• ISP specific policies: Many ISPs, including Optonline.net, have internal, unpublished policies that can cause unique deliverability anomalies, requiring persistent testing and observation.

Key considerations

• Content and header inspection: Beyond visible content, thoroughly inspect email headers and raw message source for anything that might increase size or trigger filters. Even minor issues can cause deliverability issues.
• Reputation management: Focus on maintaining an excellent sender reputation with all ISPs, as this can influence how strictly content and size limits are enforced. Learn about recovering domain reputation for more insights.
• Test variations: Systematically test sending emails to Optonline.net with varying content, subject lines, and image sizes to pinpoint what might be triggering the 554 error.
• Engage postmaster: If problems persist, attempt to contact the Optonline.net postmaster directly for clarification on their specific policies or recent system changes. Sometimes, these issues can be related to a temporary blocklist or system glitch.

Key findings

• RFC 5321 definition: RFC 5321 (SMTP) defines a 554 Permanent Negative Completion Reply as a permanent failure, often due to security or policy violations, including explicit message size limits.
• Size calculation complexity: The actual size calculation by mail servers can encompass more than just the visible content, including encoding (e.g., MIME, base64 for images) and headers, which can lead to unexpected size inflation.
• ISP variations: While RFCs set standards, individual ISPs like Optonline.net have discretion in implementing filtering rules and error message interpretations, which can differ from the literal RFC interpretation.
• Temporary vs. permanent: Standard protocols typically use 4xx codes for temporary failures. A 554 implies a permanent issue, making intermittent delivery puzzling unless other dynamic factors are at play.

Key considerations

• Review ISP policies: Consult Optonline.net's postmaster site or publicly available Acceptable Use Policy (AUP) for any specific guidelines on message size, content, or sending practices.
• Encoding overhead: Be aware that even for plain text, MIME encoding adds overhead. Rich HTML and embedded images using Base64 encoding can significantly increase the byte count, potentially pushing messages over unexpected limits.
• Error interpretation: Understand that an ISP might use a generic 554 error for various policy violations, not just literal message size issues. It's important to differentiate between stated error and actual cause.
• RFCs as a baseline: While RFCs provide a baseline, they do not dictate every aspect of an email system's behavior. Real-world implementations often have added layers of security and filtering, as described in RFC 5321.
• Authentication standards: Ensure your email authentication (SPF, DKIM, DMARC) is correctly configured, as authentication failures can sometimes lead to ambiguous rejections.