Why are Microsoft IPs blocking AWS SMTP servers?

Key findings

• IP reputation: Microsoft maintains a very aggressive filtering policy against IP ranges known for spam, and AWS's extensive IP space is frequently associated with abusive sending practices.
• Shared bad neighborhood: Even with dedicated IPs, senders on AWS can be negatively impacted by the actions of other malicious senders within the same or neighboring IP blocks. This means a single bad actor can affect the reputation of an entire subnet.
• Temporary unblocks: While Microsoft may unblock specific IPs upon request, these unblocks are often temporary, and the IPs can quickly be blocked again due to the ongoing reputation issues of the broader AWS network.
• Provider-specific challenges: This problem is not unique to Microsoft; other ISPs like Orange.fr and Yahoo Mail have also shown similar patterns of blocking AWS IPs.

Key considerations

• Understand the root cause: Recognize that the issue is often systemic to the AWS IP space rather than solely a problem with your sending practices, although maintaining high deliverability standards is always crucial.
• Monitor blocklists: Regularly check if your IPs are listed on public blocklists or if Microsoft's Sender Reputation Data (SNDS) indicates poor reputation for your IPs or ranges.
• Evaluate sending infrastructure: Consider if AWS is the most suitable platform for your email sending needs, especially for high-volume or critical communications to Microsoft domains. Specialized email service providers often manage their own IP reputation more effectively.
• Implement best practices: Ensure all standard email authentication (SPF, DKIM, DMARC) is correctly configured, as this can help mitigate some reputation issues, even if it doesn't solve the core IP problem.

Key opinions

• Widespread issue: Many marketers confirm that they face ongoing problems with Microsoft blocking AWS IPs, indicating it's not an isolated incident.
• AWS's bad reputation: There's a strong belief that Microsoft's filters are particularly harsh on AWS IP ranges due to their historical use by spammers, despite individual senders maintaining good practices.
• Collective punishment: Marketers feel they are being penalized for the actions of others sharing the same IP space, even when their own sending volume and practices are legitimate.
• Temporary solutions: While submitting unblock requests can provide temporary relief, the underlying issue persists, making it a recurring challenge.

Key considerations

• Impact on campaigns: Frequent blockages directly affect campaign performance and outreach, requiring constant monitoring and troubleshooting efforts.
• Alternative sending methods: Many marketers explore moving their email sending to platforms specifically designed for deliverability, rather than relying on general cloud infrastructure like AWS for direct SMTP sending.
• Proactive reputation management: Implementing robust list hygiene, managing sender reputation, and ensuring proper authentication (SPF, DKIM, DMARC) are crucial, even if the underlying AWS IP issue remains.
• Understanding throttling: Microsoft may also throttle emails from certain IPs, which is a precursor to outright blocking. Marketers should be aware of these subtle signs of deliverability issues. More on Microsoft throttling.

Key opinions

• Avoid direct AWS sending: A strong recommendation from experts is to avoid sending mail directly from AWS instances due to their consistently poor IP reputation.
• Spammer preference: AWS is favored by spammers because it allows for rapid provisioning of new servers and IP addresses, enabling them to quickly pivot when existing IPs are blocked.
• Aggressive Microsoft filtering: Microsoft, in particular, implements very aggressive filtering against the entire AWS IP space to combat the high volume of spam originating from it.
• Bad neighborhood effect: Even if individual IPs are clean and have good practices, they suffer from the collective negative reputation of the broader AWS network they reside within.

Key considerations

• Rethink infrastructure: For reliable email deliverability, especially to major ISPs like Microsoft, a dedicated email service provider is often a more effective solution than managing SMTP on AWS directly.
• Utilize AWS SES with caution: While AWS SES manages IP reputation internally, it's still part of the AWS ecosystem. Senders must adhere strictly to SES sending policies to maintain a good sender reputation.
• Reverse DNS configuration: Ensure that rDNS (or PTR records) are correctly configured to point back to your sending domain. While not a complete solution, it's a fundamental step for IP legitimacy.
• IP warming implications: IP warming on AWS can be particularly challenging with Microsoft domains due to their aggressive filtering. For more on this, see how to resolve IP warming issues with Microsoft.

Key findings

• Reputation-based blocking: Microsoft's systems, like others, heavily rely on reputation scores tied to IP addresses and sending domains. If a large percentage of mail from an IP range is deemed spam, the entire range can be affected.
• Error code S3140: Bounce messages with error code S3140 from Microsoft typically indicate that part of the sender's network is on their internal blocklist (or block list).
• Automated systems: Microsoft's spam filtering is highly automated, relying on real-time threat intelligence and historical data to make blocking decisions, rather than manual reviews for every IP.
• IP space considerations: Large cloud providers like AWS allocate vast IP ranges that can quickly change hands between users, making it challenging for ISPs to maintain granular reputation data for each IP within such dynamic pools.

Key considerations

• Sender reputation data: Microsoft offers tools like the Smart Network Data Services (SNDS) and Junk Mail Reporting Program (JMRP) for senders to monitor their IP health and receive feedback. These tools are crucial for understanding deliverability issues directly from Microsoft's perspective.
• Compliance with guidelines: Adhering to Microsoft's sender guidelines, including proper authentication (SPF, DKIM, DMARC), consent, and content quality, is vital for long-term deliverability.
• Delisting process: Documentation outlines the process for requesting IP delisting, often through a dedicated portal or support ticket system. However, success depends on resolving underlying sending issues.
• Dynamic IP behavior: Cloud service providers often use dynamic IP allocation, which can make it harder for ISPs to consistently track reputation, leading to more blanket blocklists for entire ranges.