Why am I seeing temporary server errors on 0365 for specific subnets?

Key findings

• Persistent Errors: Despite the "temporary" label, these errors can persist for months, leading to email drops after retry attempts are exhausted, suggesting a non-transient problem.
• Subnet Specificity: The issue is often isolated to specific sending subnets, while emails from other subnets (even the same content) deliver successfully to O365. This strongly implies an IP or network-level filtering decision.
• Microsoft's Denials: Microsoft support (via the admin panel) may deny any active blocks, which can be confusing for senders.
• Post-DATA Rejection: The error often occurs after the DATA command in the SMTP conversation, sometimes followed by a puzzling "Need mail command" error, indicating the rejection is not a simple pre-connection IP block.
• Underlying Cause: Potential causes include subtle technical misconfigurations (e.g., DKIM or SPF issues), or an unacknowledged IP reputation issue on the O365 side.

Key considerations

• Beyond Simple Blocklist: This error message doesn't directly correspond to public blocklists. Microsoft's internal filtering systems and reputation scores are more likely at play.
• MTA Configuration: Examine your Mail Transfer Agent (MTA) configuration, including EHLO values, rDNS, and TLS negotiation, as subtle differences across subnets might trigger specific O365 filters.
• Hidden Security Blocks: There's a possibility of security-related blocks at Microsoft's gateways that aren't visible through standard support channels or query tools, especially if a subnet has a history of suspicious activity. Some users have reported temporary server errors related to Office 2013 and registry keys, which, while not directly related to sending IPs, highlight the varied nature of Microsoft's transient errors.
• Exhaustive Troubleshooting: Conduct a thorough review of your sending practices and IP health for the problematic subnet. Consider it as a long-term deliverability troubleshooting exercise.

Key opinions

• Not Truly Temporary: If retries are unsuccessful for months, the error is not genuinely temporary, even if the error message suggests it is.
• IP-Specific Problems: The problem manifesting only with a specific sending subnet, while other subnets deliver the same email successfully, strongly indicates an issue related to the problematic IP range, which could be an IP blocklist issue or reputation filter.
• Microsoft's Feedback: It is a common experience that Microsoft support may deny an IP block, even when symptoms strongly suggest one.
• Comparison to Other ISPs: Some compare the situation to past experiences with other ISPs like Yahoo, where IPs could get "stuck" on their side, leading to similar persistent non-delivery issues, which aligns with the Yahoo TS errors.

Key considerations

• System Configuration: Despite Microsoft's denials, the consistent failure points to something specific about the sending server or subnet configuration that is triggering a filter, even if it's not a formal blocklist entry. This could include issues with SPF, DKIM, or rDNS.
• Data Command Timing: The rejection coming after the DATA command suggests content-related filtering or a more complex technical validation failure, rather than a simple IP block on connection.
• Use Working Subnets: While a root cause analysis is ideal, a practical interim solution is to route traffic through subnets that are successfully delivering to O365.
• Proactive Monitoring: Continuously monitor your sending IP reputation and authentication records (SPF, DKIM, DMARC) to prevent similar issues, as these factors heavily influence deliverability to large mailbox providers like Office 365.

Key opinions

• Not a Hard Block Message: The message "Temporary server error" is distinct from Microsoft's typical hard block messages, suggesting a different type of filtering or technical issue rather than a straightforward IP blacklisting.
• Technical Anomaly: Errors occurring after the DATA command, such as "Need mail command," are highly unusual and point towards an issue in the SMTP protocol negotiation or a server-side problem triggered by the specific sending IP.
• Reputation or Configuration: The cause is likely either a subtle IP reputation issue specific to that subnet or a technical misconfiguration on the sending server (e.g., related to TLS, EHLO, or rDNS) that causes Microsoft's systems to reject the mail.
• Security Gateway Blocks: There's a strong possibility that the block is a security-related block placed at Microsoft's gateways, which might not show up in typical support query tools used by their front-line staff.
• Stuck Blocks: Such blocks are often supposed to auto-expire, but sometimes they can get "stuck" on the ISP side, requiring persistent escalation with O365 support.

Key considerations

• Deep Dive into Logs: Perform in-depth SMTP traffic sniffing and analyze full logs to identify any anomalies in negotiation, such as TLS handshake issues or unexpected EHLO values, that could lead to these errors. This often reveals details not apparent from basic bounce messages.
• External Data Feeds: Investigate whether Microsoft is using external data feeds, such as Talos, for reputation, as a historical issue there could contribute to an IP block, or blocklist entry, even if the current status appears clean.
• Escalation to Microsoft: If initial support avenues fail, persistent escalation within Microsoft's O365 support is necessary to reach higher-tier teams who might have access to more detailed gateway logs or internal block mechanisms, as Microsoft's filtering can be complex.
• IP Reputation Management: Ensure your sending IP has impeccable reputation. If it's IPv6, stricter adherence to best practices is often required. For IPv4, consistent, clean sending is critical to avoid IP blocking scenarios.

Key findings

• SMTP Reply Codes: A 4xx SMTP reply code (like 451) indicates a temporary failure, suggesting the sender should retry. However, when persistent, it signals a deeper, often unstated, issue with the sending environment.
• Extended Status Codes: The "4.4.4" in the reply is an enhanced status code, often indicating network or routing problems, or general mail system issues, but not specific content or sender reputation blocks.
• ATTR3 Tag: The "ATTR3" tag is an internal Microsoft identifier that doesn't have public documentation. It implies an internal attribute or filter category contributing to the error.
• Authentication Failures: Issues with SPF, DKIM, or DMARC can lead to temporary deferrals or rejections, even if not explicitly stated in the error message, as mail servers often prioritize authentication. Understanding how these protocols work is essential.

Key considerations

• RFC Compliance: Ensure your MTA fully adheres to SMTP RFCs (e.g., RFC 5321) regarding command sequences (like MAIL FROM, RCPT TO, DATA). Errors like "Need mail command" can sometimes indicate a protocol violation, even if subtle. For example, some SMTP nuances are described in articles on issues with SPF records not defining an IP pool.
• IP and Domain Reputation: Microsoft's filtering relies heavily on IP and domain reputation. A low reputation score for a specific subnet can trigger these "temporary" errors, effectively acting as a soft block or rate limit, even without being listed on a public blacklist. This can be complex, as even SPF TempErrors can contribute to overall reputation.
• Transport Layer Security (TLS): Ensure proper TLS negotiation and certificate validation, as failures here can lead to connection drops or errors interpreted as temporary server issues by the receiving MTA.
• Mail Flow Rules and Connectors: While this error is usually on the sender's side, administrators of the recipient O365 tenant should also review their mail flow rules, connectors, and security policies, which might inadvertently affect specific sending IPs.