How to troubleshoot Postfix 'too many connections' errors after upgrading?
Summary
What email marketers say12Marketer opinions
Email marketer from Geekflare explains that overly aggressive greylisting can cause senders to retry frequently, increasing the number of connections. Review your greylisting configuration.
Email marketer from EmailAdminBlog.com recommends checking and adjusting Postfix's `initial_destination_concurrency`. They explain that a low initial concurrency setting could lead to quicker exhaustion of available connections.
Email marketer from LinuxBabe.com responds with the recommendation to monitor the Postfix queue using `mailq` and `postqueue -p`. This allows you to identify if there's a backlog of messages causing the connection spikes.
Email marketer from DigitalOcean responds that it's important to verify that the firewall isn't limiting the number of outbound connections from the Postfix server. He shares that iptables or firewalld could be restricting connections.
Email marketer from Reddit suggests verifying the `smtp_tls_connection_reuse` setting. He responds that if this is disabled, connections might not be reused efficiently, leading to more connections being created.
Email marketer from StackOverflow responds that high system load can cause Postfix to become slow and lead to connection issues. Monitor CPU, memory, and disk I/O.
Email marketer from ServerFault recommends checking the `maxproc` setting in `master.cf`. He explains that limiting the number of processes can lead to this error, especially if the workload has increased.
Email marketer from LinuxQuestions.org explains that DNS resolution issues can cause delays and Postfix might try to open more connections than necessary. Ensure DNS is resolving quickly and correctly.
Email marketer from MXToolbox says to ensure your server has a properly configured reverse DNS (PTR) record. Some receiving servers might delay or reject connections from servers without valid PTR records.
Email marketer from Ubuntu Forums shares a configuration setting for TLS. He explains that incorrect TLS settings can cause connection problems. Review `smtp_tls_security_level` and related parameters.
Email marketer from StackExchange recommends checking the system's resource limits (ulimit). He explains that if the Postfix process is hitting the maximum number of open files or processes, it can manifest as connection errors.
Marketer from Email Geeks suggests that the #postfix channel on libera.chat (IRC) has been helpful in the past.
What the experts say4Expert opinions
Expert from Spam Resource explains that aggressive greylisting combined with a short retry window can increase connection attempts and trigger 'too many connections' errors in Postfix.
Expert from Email Geeks explains that `smtpd_junk_command_limit` is an inbound mail setting and wouldn't affect outbound concurrency.
Expert from Email Geeks suggests checking the configuration file and the exact errors received. He also suggests asking the postfix-users mailing list for assistance.
Expert from Word to the Wise explains that slow or unreliable DNS resolution can cause Postfix to retry connections more often, potentially leading to 'too many connections' errors. He shares to always check DNS settings and ensure fast resolution times.
What the documentation says3Technical articles
Documentation from Postfix.org explains that the `default_process_limit` parameter controls the maximum number of concurrently running Postfix processes. Increasing this might help, but also check resource usage.
Documentation from Postfix.org explains the use of `destination_rate_delay` parameter. He shares that this might be needed if the upgraded Postfix version is more aggressive, causing the destination server to see too many connections.
Documentation from Postfix.org shares that the `transport_destination_concurrency_limit` (or specific transport like `smtp_destination_concurrency_limit`) limits the number of concurrent connections to a single destination. Review if this value is too restrictive.