Summary
A range of solutions exist for managing email throttling and load balancing ahead of the MTA. These include commercial platforms (Ongage, SparkPost, PMTA, GreenArrow Engine, MailerQ) that offer built-in capabilities. Load balancers like HAProxy and Nginx can distribute traffic across multiple Postfix instances, while message queue systems like RabbitMQ and Amazon SQS can buffer emails to prevent overwhelming the MTA. Traffic shaping with tools like 'iproute2' combined with Postfix, configuring multiple queues with transport maps, and using multiple public IP addresses are also effective. Monitoring tools like MXToolbox can identify performance bottlenecks. Crucially, feedback loops and complaint rates should inform throttling strategies, and maintaining a solid sending reputation is paramount.
Key findings
- Commercial Platforms: Platforms like Ongage, SparkPost, PMTA, GreenArrow Engine, and MailerQ provide integrated throttling and load balancing.
- Load Balancers: HAProxy and Nginx distribute SMTP traffic across multiple MTAs using various algorithms and health checks.
- Message Queues: RabbitMQ and Amazon SQS decouple sending, buffering emails and allowing controlled release rates.
- Postfix Configuration: Multiple Postfix instances or configurations with queues and maps enable traffic management.
- Multiple IPs: Distributing sending across multiple IPs aids load management and improves deliverability.
- Feedback and Reputation: Real-time feedback integration and reputation management are vital for adjusting strategies.
Key considerations
- Implementation Location: Throttling should occur at the MTA level to be effective.
- Skill Requirements: In-house solutions demand specific technical skills, particularly with Postfix and networking.
- Configuration Complexity: Ensuring correct configurations is vital for effective operation and reliability.
- Infrastructure Cost: Additional infrastructure (multiple MTAs, queuing systems) may increase costs.
- Reputation Monitoring: Continual reputation monitoring is essential for deliverability optimization.
- Feedback Integration: Systems should adapt dynamically based on real-time feedback and engagement.
What email marketers say
12 marketer opinions
Several solutions exist for throttling and load balancing email traffic in front of the MTA. These include commercial platforms like Ongage, SparkPost, GreenArrow Engine, PMTA, and MailerQ; load balancers such as HAProxy and Nginx used in conjunction with multiple Postfix instances; message queue systems like RabbitMQ and Kafka; and traffic shaping with tools like iproute2. In-house solutions involving Postfix configurations with multiple queues and transport maps, and using multiple public IP addresses are also viable. Monitoring tools like MXToolbox can help identify bottlenecks.
Key opinions
- Commercial Platforms: Platforms like Ongage and SparkPost offer built-in throttling and load balancing capabilities.
- Load Balancers: HAProxy and Nginx can distribute SMTP traffic across multiple Postfix instances.
- Message Queues: RabbitMQ and Kafka can buffer emails to prevent overwhelming the MTA.
- Postfix Configuration: Postfix can be configured with multiple outbound queues and transport maps for traffic shaping.
- Multiple IPs: Using multiple public IP addresses can improve deliverability.
- Monitoring: Tools like MXToolbox can help identify performance bottlenecks.
Key considerations
- MTA Location: Throttling needs to occur at the MTA level, not just at the injection point.
- Skill Set: In-house solutions require specific technical skills, particularly in Postfix configuration or networking.
- Learning Curve: Commercial solutions often have a learning curve and require system integration.
- Underlying Networking: Understanding networking is important for any system.
- Domain Reputation: Domain reputation is important for mail deliverability.
- Feedback Loops: You should adjust sending volume based on complaint rates.
Marketer view
Email marketer from Stack Overflow suggests using a load balancer like HAProxy in front of multiple Postfix instances. The load balancer distributes the load, and Postfix handles the actual mail delivery.
7 Oct 2021 - Stack Overflow
Marketer view
Marketer from Email Geeks explains that throttling should be done in the MTA to avoid only throttling the injection rate, but not the deliveries.
3 Jan 2023 - Email Geeks
What the experts say
2 expert opinions
Experts emphasize controlling email volume using applications in front of the MTA and adjusting sending based on feedback loops and complaint rates. Managing reputation is also considered critical.
Key opinions
- Volume Control: Applications in front of the MTA can control email volume effectively.
- Feedback Loops: Throttling should be dynamically adjusted based on feedback loops and complaint rates.
- Reputation Management: Maintaining a good reputation is essential for email deliverability.
Key considerations
- Real-time Adjustment: The application needs to be capable of real-time adjustments based on receiver responses.
- Reputation Monitoring: Implement processes to monitor and manage sender reputation proactively.
- Comprehensive Metrics: Take into account metrics such as hard bounces, spam complaints, and engagement rates.
Expert view
Expert from Spamresource.com recommends controlling email volume with an application that sits in front of the MTA to ensure that you aren't sending too much mail. Spamresource.com also recommends managing reputation.
6 Mar 2025 - Spamresource.com
Expert view
Expert from Word to the Wise explains that throttling should consider feedback loops and complaint rates, adjusting sending volume based on receiver responses to avoid deliverability issues. This implicitly addresses the load and throttling challenges.
11 Aug 2021 - Word to the Wise
What the documentation says
5 technical articles
The documentation suggests several approaches for throttling and load balancing email traffic. HAProxy and Nginx can function as TCP load balancers, distributing traffic to multiple MTAs. Postfix can be configured for load balancing across multiple instances. Message queues like Amazon SQS and RabbitMQ can act as buffers, decoupling sending from the application and enabling rate limiting.
Key findings
- HAProxy/Nginx as Load Balancers: HAProxy and Nginx are suitable for distributing SMTP connections across multiple MTAs, offering various load balancing algorithms and health checks.
- Postfix Load Balancing: Postfix can be configured in a load-balanced setup with multiple instances behind a load balancer.
- Message Queues as Buffers: Amazon SQS and RabbitMQ act as buffers to manage and throttle email sending rates, decoupling the sending process.
Key considerations
- Configuration Complexity: Proper configuration is crucial for each solution to ensure optimal performance and mail delivery.
- Infrastructure Requirements: Implementing these solutions might require additional infrastructure, such as multiple MTAs or queueing systems.
- Monitoring: Ongoing monitoring is necessary to ensure proper functioning and identify potential issues.
Technical article
Documentation from HAProxy.org explains that HAProxy can be configured to load balance TCP traffic, making it suitable for distributing SMTP connections across multiple MTAs. It provides various load balancing algorithms and health checks.
6 Jul 2023 - HAProxy.org
Technical article
Documentation from RabbitMQ explains that this message broker can act as a buffer in front of your MTA to smooth out traffic spikes, allowing you to throttle the rate at which messages are delivered to the MTA.
28 Nov 2021 - RabbitMQ
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