Summary
ESPs can stress test their email systems and MTA servers using a combination of specialized tools, MTA configuration, and simulation techniques. SMTP-sink is recommended for efficient load testing by discarding inbound mail, unlike Mailhog. Generating realistic email traffic via shell scripts, CSV files, and synthetic data is essential. Simulating real-world conditions includes varying sending volumes, recipient engagement, and abuse complaints. Monitoring key metrics (queue length, delivery rates, CPU utilization), analyzing bounce codes, and testing authentication failures are crucial. Monitoring tools like Nagios and New Relic help identify bottlenecks. Rendering tests should ensure consistent display across clients. MTAs like Postfix and Exim offer configuration options for high-volume testing and traffic simulation. RFC 5321 compliance is vital. Oracle and Microsoft provide tools for user profile simulation and performance measurement. Finally, ensure that SMTP sinks simulate tempfails as a normal part of the system operation and do not become a bandwidth test.
Key findings
- SMTP-sink efficiency: SMTP-sink is more efficient than Mailhog for load testing.
- Realistic traffic simulation: Simulating real-world email traffic patterns is vital.
- Comprehensive monitoring: Monitoring key metrics and bounce codes is essential.
- Authentication testing: Testing authentication failures is critical for deliverability.
- Configuration options: MTAs offer configuration options for high-volume testing.
- SMTP sinks simulate tempfails: SMTP Sinks should simulate tempfails as part of the operations.
Key considerations
- Tool selection: Select specialized tools like SMTP-sink for efficient testing.
- Data generation: Use scripts, CSV files, and synthetic data for realistic traffic.
- Traffic simulation: Vary sending volumes, engagement, and simulate abuse.
- Metric tracking: Monitor queue length, delivery rates, and CPU utilization.
- Compliance: Ensure RFC 5321 compliance.
- User Simulation: Simulate virtual user profiles and behavior.
- Simulate realistic results: Tempfails help create realistic results as part of load testing.
What email marketers say
8 marketer opinions
ESPs can stress test their email systems and MTA servers using various tools and methods to identify bottlenecks and vulnerabilities. Simulating real-world email traffic patterns, including varying volumes and recipient engagement, is crucial. Employing synthetic data, simple scripting tools, and specialized tools like MXToolbox can generate high email volumes for testing. Monitoring key metrics such as message queue length, delivery rates, and server CPU utilization, alongside bounce code analysis, provides insights into system performance. Using monitoring tools like Nagios or New Relic during testing is essential for identifying bottlenecks. Additionally, considering potential authentication failures and performing rendering tests under load are important to ensure deliverability and prevent abuse.
Key opinions
- Realistic Simulation: Simulating real-world traffic is vital for identifying vulnerabilities.
- Metric Monitoring: Monitoring key metrics provides insights into system performance.
- Synthetic Data: Synthetic data can effectively simulate realistic email traffic patterns.
- Bottleneck Identification: Monitoring tools are essential for pinpointing bottlenecks during stress tests.
- Authentication Failure: Testing for authentication failures is important for deliverability.
- Tempfails Required: MTAs need to simulate tempfails so that a bandwidth test does not occur.
Key considerations
- Traffic Patterns: Simulate varying email volumes and recipient engagement levels.
- Tools Variety: Utilize a mix of simple scripts and specialized tools for testing.
- Monitoring Tools: Implement monitoring tools to track resource utilization and identify bottlenecks.
- Rendering Tests: Ensure email rendering is consistent across different clients under load.
- Abuse Handling: Feedback loops and complaint handling mechanisms should be stress tested
Marketer view
Email marketer from Reddit shares that simple scripting tools can be used to generate and send a large volume of emails to stress test an email system. This approach can help identify performance bottlenecks and limitations in the system's ability to handle high-volume sending.
12 Oct 2023 - Reddit
Marketer view
Email marketer from StackOverflow explains that during a stress test, using monitoring tools such as Nagios or New Relic is essential for finding bottlenecks. These monitoring tools would help indicate which resources are being stretched to their limits.
18 May 2025 - StackOverflow
What the experts say
8 expert opinions
ESPs can effectively stress test email systems by utilizing tools like smtp-sink (or smtpsink) for load testing, which cheaply discards inbound mail, unlike Mailhog. Generating CSV files of email addresses via shell scripts and simulating customer behavior are also helpful. Smtp-sink provides options for delays and rejections. Varying message content and structure, simulating abuse complaints, analyzing bounce codes, and testing authentication failures (SPF, DKIM) are crucial to expose vulnerabilities. Web load testing tools can supplement email-specific testing. These tests should be used to identify weaknesses in feedback loops, complaint handling, and the overall MTA server setup.
Key opinions
- SMTP-Sink Advantage: SMTP-sink is more efficient for load testing than Mailhog due to discarding inbound mail.
- Behavior Simulation: Simulating customer behavior helps create realistic stress tests.
- Abuse Complaint Testing: Simulating abuse complaints helps evaluate feedback loops.
- Content Variation: Varying message content uncovers handling weaknesses.
- Authentication Importance: Testing authentication failures prevents deliverability issues.
- Bounce Code Insights: Analyzing bounce codes helps optimize MTA configuration.
Key considerations
- Tool Selection: Choose tools designed for load testing (like smtp-sink).
- Realistic Data: Use shell scripts and CSV files to generate realistic email data.
- Traffic Simulation: Incorporate options like delays and rejections in testing.
- Content Diversity: Vary message types, attachments, and HTML formatting.
- Failure Scenarios: Test for SPF and DKIM authentication failures.
- Feedback Loop: Check if system has feedback loops that capture abuse complaints
Expert view
Expert from Email Geeks recommends using smtp-sink or smtpsink for a mail server that accepts and discards mail, suggesting installing it on owned servers, pointing an MX record to them, and loading up a spam cannon to stress test. He also adds that regular web load testing tools can be used for other aspects.
14 May 2023 - Email Geeks
Expert view
Expert from Email Geeks suggests generating CSV files of email addresses using a shell script and loading them to simulate customer behavior for stress testing.
23 Apr 2022 - Email Geeks
What the documentation says
5 technical articles
ESPs can utilize the configuration options within MTAs such as Postfix and Exim to stress test their email systems. Postfix allows for generating high email volumes and optimizing sending rates, while Exim provides rate limiting and queue management features to simulate various traffic conditions and abuse scenarios. RFC 5321 compliance testing is crucial for interoperability. Tools like Oracle Communication Messaging Server and Microsoft Exchange Server's Jetstress and LoadGen provide methods for creating virtual user profiles, measuring response times, throughput, and assessing disk subsystem performance.
Key findings
- MTA Configuration: MTAs (Postfix, Exim) offer configuration options for high-volume testing.
- Traffic Simulation: Exim allows simulating traffic conditions and potential abuse.
- Protocol Compliance: SMTP compliance (RFC 5321) is critical for interoperability.
- Performance Measurement: Oracle provides tools for generating user profiles and measuring server performance.
- Tool Sets: Microsoft Exchange provides toolsets (Jetstress, LoadGen) for system testing.
Key considerations
- Sending Rate Optimization: Optimize sending rates using MTA configurations during testing.
- Queue Management: Use queue management features to simulate different traffic loads.
- Abuse Scenario Testing: Assess system handling of potential abuse scenarios.
- Resource Utilization: Measure response times, throughput, and resource utilization.
- Disk Subsystem: Evaluate the performance of the disk subsystem before production (Jetstress).
- SMTP Standards: Comply with SMTP standards at high loads.
Technical article
Documentation from Postfix.org details how Postfix can be configured to generate high volumes of test emails, allowing for the evaluation of MTA server performance under stress. The documentation outlines settings for optimizing sending rates and monitoring queue behavior during testing.
8 Apr 2024 - Postfix.org
Technical article
Documentation from Oracle Communication Messaging Server explains how to use it to conduct load testing of the server. The Oracle documenation highlights how to generate virtual user profiles to simulate real user behavior and how to measure response times, throughput, and resource utilization.
5 Mar 2025 - Oracle
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