How can I change the retransmission time in Postfix?

Key findings

• Primary parameter: The transport_retry_time parameter in the main.cf file directly controls the time between attempts by Postfix to contact a malfunctioning delivery transport. Its default value is 60 seconds.
• Queue scan frequency: The queue_run_delay setting (default 300 seconds or 5 minutes) determines how often the Postfix queue manager scans the deferred queue for messages to retry, influencing the overall retransmission behavior.
• Greylisting impact: Greylisting often results in a temporary 4xx SMTP error, causing the mail to be deferred. Understanding how greylisting works is key to configuring Postfix retries effectively, as premature retries can reset the greylisting timer.
• Exponential backoff: Postfix typically employs an exponential backoff strategy for retries of deferred mail, meaning retry intervals increase over time to reduce load on recipient servers and account for longer outages.

Key considerations

• Balance of speed and compliance: While you can adjust retry times, shortening them excessively for greylisting can be counterproductive, as the recipient server will keep rejecting the connection until its greylisting period expires. This could lead to your IP being rate-limited or even blocklisted if seen as aggressive.
• Impact on deliverability: Frequent temporary errors can indicate underlying issues, which affect your overall email deliverability. Understanding and interpreting SMTP error codes is essential before making changes.
• Configuration file: Changes to Postfix retry behavior are made in the /etc/postfix/main.cf file. Always back up your configuration before making modifications.
• Default values: Postfix's default retry behavior, as detailed in its official documentation, is often optimized for general use. Deviating too much from defaults without a clear reason can introduce new problems.

Key opinions

• Greylisting challenges: Many marketers note that greylisting is a common cause of temporary deferrals, affecting all MX records for a destination domain, which can be frustrating.
• Greylisting resets: There's a concern that retrying too quickly might reset the greylisting timer on the recipient server, indefinitely prolonging delivery delays.
• Managing multiple MTAs: Administrators managing several mail transfer agents (MTAs) like Postfix, Exim, and PowerMTA can find it overwhelming to keep track of specific configuration details for each.
• Trial and error: Some users opt for testing different Postfix settings through trial and error to observe their impact on email delivery and greylisting responses.

Key considerations

• Understanding delays: It is important to determine if the Postfix server is waiting for the full greylisting period (e.g., 300 seconds) before attempting a retransmission. This directly impacts slow email delivery.
• Configuration complexity: Modifying Postfix can be intricate due to its extensive configuration options, making precise adjustments challenging without deep knowledge of its parameters, particularly for issues like email throttling.
• Server response codes: Marketers should monitor 4xx SMTP response codes (temporary failures) to understand if the issue is indeed greylisting or another temporary condition that requires deferral and retransmission.
• Postfix retry behavior: The default Postfix retry behavior, including parameters like queue_run_delay, typically waits 5 minutes before checking the deferred queue and attempting to resend, as noted on Spiceworks Community.

Key opinions

• Careful Postfix usage: Some experts avoid using Postfix for specific operational reasons, or they help others transition away from it, which suggests that while powerful, it might not be the ideal solution for every scenario without extensive expertise.
• 4xx vs. non-responsive MTA: A key distinction for experts is whether a 4xx SMTP response (temporary failure) is truly a 'transport malfunction' in Postfix's definition. This influences whether transport_retry_time or other queue-related settings are more relevant for deferrals.
• Postfix auto-pausing: Experts note that Postfix might not have an inherent 'auto-pausing' mechanism for specific deferral types without custom modules or careful tuning of its queue parameters.
• Documentation accuracy: While Postfix documentation is comprehensive, experts sometimes find that the precise interaction of parameters for complex scenarios like greylisting requires practical testing and deeper analysis.

Key considerations

• Distinguishing deferrals: It's vital for experts to differentiate between connection timeouts (e.g., connection timeout errors) and soft bounces due to greylisting. Each requires a slightly different approach to retry configuration.
• Queue management: Experts advise that tweaking queue_run_delay, minimal_backoff_time, and maximal_backoff_time is often more impactful for general deferrals than just transport_retry_time.
• Impact on rate limits: Aggressive retry settings can cause a sender to hit rate limits imposed by mailbox providers, which can lead to longer deferrals or even outright blocking. Experts advise adhering to established best practices.
• Reputation implications: From an expert perspective, constant temporary deferrals can negatively impact sender reputation, even if mail eventually gets delivered. It's better to address the root cause of deferrals than just adjust retry timers.

Key findings

• Parameter purpose: The Postfix postconf manpage clearly defines transport_retry_time as the interval between attempts to contact a malfunctioning delivery transport, with a default of 60 seconds.
• Queue processing interval: Documentation specifies queue_run_delay (default 300s) as the frequency at which the queue manager processes deferred mail. This influences when messages are re-evaluated for sending.
• Minimum and maximum backoff: The minimal_backoff_time (default 500s) and maximal_backoff_time (default 4000s) define the bounds for exponential backoff used for deferred queue processing.
• Time units: The documentation confirms that time units (s, m, h, d, w) can be used for these parameters, with seconds being the default unit, allowing for flexible configuration.

Key considerations

• Interplay of settings: Documentation emphasizes that various queue parameters interact, and changing one without considering others can lead to unexpected mail delivery behavior or increase server load unnecessarily. This is especially true when dealing with issues like connection timeout errors.
• Error handling: The documentation clarifies how Postfix interprets different SMTP error codes (e.g., 4xx vs. 5xx) and how these dictate whether an email is deferred for retransmission or bounced permanently.
• Default philosophy: Postfix's default settings for retry and queue management are designed for robustness and compliance with standard email practices, including respecting temporary deferrals like those from greylisting.
• Logging and monitoring: Official guides often stress the importance of monitoring Postfix logs to observe the actual retransmission behavior and identify patterns of deferrals, which can help in diagnosing deliverability issues.