Do US email providers throttle email based on sender IP geolocation?

Key findings

• Observed throttling: Some senders have reported experiencing throttling, such as concurrent connection errors, specifically when sending from non-US IP addresses to US providers like Cox, despite adhering to stated connection limits.
• IP geolocation as a factor: Conversations with ISP representatives indicate that IP geolocation can influence how an email provider manages incoming traffic, potentially leading to increased throttling for geographically distant senders.
• Traffic management: Providers typically receive a higher volume of legitimate email traffic from their own country and surrounding regions. Throttling based on geolocation might be a method to adjust sending limits based on expected traffic patterns and identify anomalous behavior.
• Spam mitigation: While not the sole reason, geolocation checks could be a component of a broader strategy to mitigate botnet-based spam runs, particularly if certain regions are known for higher spam activity.
• Impact on deliverability: Shifting to US-based IPs for US recipients has reportedly led to a significant decrease in transient errors and improved deliverability for some senders, suggesting a tangible impact of IP geolocation.

Key considerations

• Unpublished policies: Most ISPs do not explicitly publish policies regarding IP geolocation-based throttling. This makes it challenging for senders to proactively optimize their sending infrastructure.
• Infrastructure complexity: For global senders, implementing regional IP pools specifically for certain countries like the US can add significant complexity to their email sending architecture. However, it can align with best practices for IP warming and deliverability.
• Sender reputation: Geolocation is likely one of many factors, alongside sender reputation, email volume, and engagement metrics, that ISPs use to determine throttling. Focusing on overall deliverability health remains paramount. Learning acceptable email sending speeds is crucial.
• Regional compliance: Differences in email regulations (e.g., CAN-SPAM versus GDPR) might influence how ISPs perceive traffic from different regions, potentially contributing to varied throttling behaviors.

Key opinions

• Experiential evidence: Many marketers report observing throttling when sending from non-US IPs to US providers, despite meeting typical sending limits. This suggests an unstated geographical preference.
• Practical solutions: To mitigate observed throttling, some marketers have successfully implemented separate IP pools or adjusted IP geolocation for US-bound traffic.
• Global sender challenges: For companies with a global client base, splitting IP ranges by country for US-specific traffic can introduce significant logistical complexity.
• Compliance differences: There's a perception among marketers that US email marketing practices can be less stringent due to CAN-SPAM regulations compared to GDPR, potentially influencing how US ISPs view foreign traffic.
• Reputation focus: Marketers are primarily concerned with maintaining sender reputation and minimizing bounce rates, which are direct indicators of deliverability health.

Key considerations

• Regional IP pooling: For organizations sending significant volumes to the US from non-US IPs, investing in dedicated IPs with US geolocation may become a necessary strategy to avoid throttling.
• Monitoring transient errors: Closely monitoring bounce logs for transient errors, especially those related to concurrent connections or rate limiting, can provide early indicators of geolocation-related throttling.
• Sender reputation first: While geolocation might play a role, marketers should prioritize maintaining a strong sender reputation through good list hygiene, relevant content, and engaged subscribers. This will always be the primary driver of inbox placement.
• ISP communication: Direct communication with ISPs, even if their policies are not public, can sometimes yield insights into specific throttling behaviors.
• Warm-up strategies: If new IPs are acquired, proper IP warming strategies are essential, regardless of geolocation, to build trust with mailbox providers.

Key opinions

• Botnet mitigation: Experts hypothesize that geolocation-based throttling could be an ISP's attempt to mitigate spam originating from botnets, especially if certain geographical regions are known for such activity.
• Regional traffic patterns: Mailbox providers often observe higher volumes of legitimate email traffic from their own country. Throttling non-local IPs could be a strategy to manage these expected traffic flows.
• Confidential policies: Many hosted email services or ISP policies, like those concerning Cloudmark, may include checks on sender location to set suitable sending limits, but these are often confidential.
• Uncommon in US (historically): While geo-restrictions are common in regions like APAC (e.g., China), experts have historically found it less common for US providers to overtly limit mail based solely on EU geolocation.
• Reputation is key: Ultimately, an ISP's decision to throttle or block email is heavily influenced by the sender's reputation, which includes factors like bounce rates and spam complaints. This impacts the overall sender reputation.

Key considerations

• Data center placement: Having data centers and sending IPs geographically aligned with target audiences can be beneficial, especially considering legal requirements around data retention, though email IP geolocation is distinct from data storage location.
• Holistic deliverability: While geolocation may be a factor, it is crucial to focus on a holistic deliverability strategy, including proper authentication (SPF, DKIM, DMARC), content quality, and list hygiene. Understanding how ISPs track engagement is critical.
• Understanding ISP algorithms: ISP filtering algorithms are complex and constantly evolving, incorporating numerous signals to assess legitimacy. Geolocation is likely one such signal among many.
• Mitigating throttling for Microsoft: For specific providers, like Microsoft, understanding their unique throttling mechanisms is key. Experts advise specific strategies to improve deliverability to such domains, even if geolocation plays a minor role. Refer to why emails to Microsoft domains are throttled.

Key findings

• Focus on reputation: Official ISP guidelines consistently emphasize that sender reputation, built on factors like low spam complaints and bounce rates, is the primary driver of deliverability and throttling decisions.
• Standard limits: Documentation often details specific connection and rate limits that all senders are expected to adhere to, regardless of their IP's location.
• Anomalous traffic detection: ISP systems are designed to detect unusual traffic patterns, which could include unexpectedly high volumes from regions not typically associated with certain sending behaviors or recipient bases.
• Fraud prevention: Geolocation can be one of many data points used in fraud detection and preventing malicious activity, which indirectly affects deliverability for suspicious sources.
• Technical compliance: Emphasis is placed on proper email authentication (SPF, DKIM, DMARC) and adherence to RFC standards, which are universal and do not depend on IP geolocation.

Key considerations

• Indirect influence: While not explicitly stated, geolocation may indirectly influence throttling as part of an ISP's overall risk assessment or traffic management strategy.
• Holistic reputation management: Senders should focus on building a robust sender reputation through consistent positive sending behaviors, as this is the most critical factor for deliverability to any provider. Understanding Gmail's bulk sending limits and their factors is a good example.
• Adherence to known limits: Always comply with published ISP connection and rate limits. If a sender is hitting these limits, it's typically due to volume or reputation issues, rather than just geolocation. Review how to handle rate limits.
• IP warm-up importance: Any new IP, regardless of its location, needs a proper warm-up period to establish trust with mailbox providers, as documented by various email service providers.
• Transparency limitations: Expect limited transparency from ISPs regarding their exact filtering and throttling algorithms, including the specific weighting of geolocation. This means observation and adaptation are key.