How to handle email sending rate and connection limits from mailbox providers?

Key findings

• Postfix configuration: Limits (such as smtpd_client_connection_count_limit and smtpd_client_recipient_rate_limit) are often set at the SMTP server level, using configurations like Postfix, not by client-side applications.
• Dynamic adjustments: Some systems use stress adaptive controls to dynamically adjust limits (e.g., from 5 connections to 1) when the server experiences high load or network port saturation. This can lead to unpredictable throttling.
• Impact of high volume: Sending a large volume of emails (e.g., over 12,000 recipients per hour from a single IP to a specific provider) can quickly exceed these limits, especially if the recipient server is under stress.
• Limited customisation: Mailbox providers using standard configurations like Postfix often do not have an easy way to adjust these limits on a per-sender basis, making compliance with set rates essential.
• Error indicators: Errors like 421 4.7.0 too many connections are direct indicators of hitting connection limits and necessitate immediate action to reduce sending speed or concurrent connections.

Key considerations

• Monitor delivery errors: Regularly review bounce messages and delivery logs for signs of throttling or connection limits (e.g., 421 errors) to identify when adjustments are needed.
• Implement adaptive throttling: Adjust your sending rate dynamically based on feedback from mailbox providers to avoid hitting hard limits. This means slowing down when you encounter rejections or delays.
• Limit concurrent connections: Configure your sending infrastructure to limit the number of simultaneous connections to specific mailbox providers, particularly if their documented or observed limits are low. For example, some may only allow five concurrent connections.
• Diversify sending IPs: For very large sending volumes to a single domain or provider, consider distributing traffic across multiple sending IPs to avoid concentrating volume that could trigger rate limits on a single IP.
• Segment and warm up: For new IPs or large lists, gradually warm up your IP by sending smaller volumes initially and slowly increasing, helping mailbox providers build trust in your sending reputation.
• Check Postfix documentation: Refer to Postfix documentation for a deeper understanding of how these parameters work and what they mean for your sending behavior.

Key opinions

• Limits are common: Many marketers face issues with mailbox provider limits, often struggling to get clear guidance or specific numbers from the providers themselves.
• Unpredictable stress modes: The inability to predict when a mailbox provider will enter a 'stress' mode and impose stricter limits is a major pain point, leading to unexpected throttling.
• High volume challenges: Sending large campaigns, especially when a significant portion of subscribers are on a single domain (like inbox.lv), makes hitting these limits almost inevitable.
• Negotiation difficulties: Marketers find it difficult, if not impossible, to negotiate higher sending limits directly with mailbox providers, especially smaller ones.
• Error messages indicate issues: Direct error responses like 'too many connections' confirm that the limits are being breached and require immediate attention.

Key considerations

• Send slower: A primary strategy is to reduce the sending speed of campaigns, spreading out sends over a longer period to stay within limits.
• Limit concurrent connections: Explicitly configure sending systems to cap concurrent connections to specific domains to prevent over-connection errors. This helps to manage concurrent outbound connections.
• Segment email lists: Divide large email lists into smaller segments and schedule sends across different days or times to manage volume, which can also improve engagement.
• Consider more IPs: For domains with extremely high subscriber counts, adding more sending IPs for that specific domain might distribute traffic enough to mitigate limits, but this needs to align with IP warming best practices.
• Optimize sending speed: Implement queue management or API controls to optimize sending speed and avoid rate limits, improving overall deliverability and process efficiency.
• Understand provider-specific nuances: Be aware that popular email providers have specific sending limits, and taking steps to understand and adhere to them can help avoid hitting these boundaries.

Key opinions

• Postfix syntax is common: Many mailbox providers, including smaller ones, use standard Postfix configurations for their SMTP server limits.
• Limits are non-negotiable: Once set, these limits are generally the volumes senders must comply with, and providers are unlikely to adjust them on a per-sender level.
• Stress mode implications: A server entering 'stress mode' indicates it's heavily overloaded, which should be a rare occurrence for major mailbox providers. Frequent stress mode suggests deeper infrastructure issues on their end, or severe abuse.
• Reputation impacts limits: While not explicitly stated in the thread, experts generally agree that a strong sender reputation can lead to more lenient limits or less frequent throttling from ISPs.
• Observing behavior: Since many ISPs do not publish specific rate limits, senders must learn and adapt to them through careful observation of bounce messages and delivery delays.

Key considerations

• Compliance is paramount: Accept that the published or inferred limits are the rules you must follow. Sending more than 12,000 recipients per hour from a single IP to a stressed server will result in errors.
• Proactive throttling: Implement internal systems to automatically detect when a provider starts throttling and reduce your send rate to that specific domain for a period. This proactive approach helps manage email sending speeds.
• Hard limit concurrent connections: Configure your Mail Transfer Agent (MTA) to strictly adhere to the maximum allowed concurrent connections for specific domains, typically 5 in many Postfix setups.
• Monitor server health indicators: While you can't predict when a remote server will be stressed, consistent observation of your email deliverability metrics can give clues to impending issues.
• Build strong sender reputation: Focus on maintaining a good sender reputation, as this is the most effective long-term strategy for achieving higher sending limits and less stringent throttling.
• Technical server management: Understand that the limits are often handled by the SMTP server itself, and not necessarily by client-side scripts, requiring server-level rate-limiting solutions.

Key findings

• Configurable limits: Mail server software allows administrators to set specific limits on concurrent connections and recipient rates per client, directly impacting how much email a sender can push through.
• Stress adaptation: Servers can be configured to dynamically reduce limits when under load, a mechanism designed to prevent system failure rather than to intentionally block legitimate mail.
• Error codes indicate limits: Specific SMTP error codes, such as 421, are used to signal when a sender has exceeded these established connection or rate limits.
• Server-side control: The imposition of sending limits is typically handled by the recipient's SMTP server, meaning senders must adjust their outbound practices to align with these remote server settings.
• Throttling as optimization: Email throttling is viewed as a technique for optimizing sending speed, ensuring that emails are delivered reliably by not overwhelming recipient servers and avoiding spam filters.

Key considerations

• Consult technical specifications: Familiarize yourself with the documentation for common MTAs like Postfix to understand the parameters governing connection and rate limits.
• Adhere to dynamic limits: Design your sending system to respond to temporary rejections and dynamically reduce sending speed, particularly when recipient servers signal stress or overload.
• Implement robust error handling: Ensure your sending platform correctly interprets and responds to SMTP 4xx errors, implementing retry mechanisms with appropriate delays.
• Consider queue management: Utilize advanced queue management or API features if available (e.g., Mailgun's API) to gain granular control over sending volume and speed per ISP.
• Prioritize deliverability: Recognize that throttling, while inconvenient, is a mechanism to maintain overall email ecosystem health and that respecting it is crucial for boosting email deliverability rates.
• Stay informed on best practices: Continuously update your knowledge on email sending speed best practices to navigate evolving ISP policies.