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Postfix does not have a universal two-emails-per-minute ceiling. That rhythm usually means a local SMTP client rate limit is set to a small value, a service-specific override is active, or messages are being accepted and then delayed in the queue or at the recipient server. Start by proving which stage is slow: SMTP acceptance, queue scheduling, or remote delivery. Then change only the setting that the logs identify.
What a two-per-minute pattern actually tells you
Postfix’s documented per-client controls cover message-delivery requests, new connections, simultaneous connections, recipients, new TLS sessions and, in newer releases, AUTH commands. The main message, connection and recipient rate limits default to 0, meaning unlimited, so a repeatable two-per-minute result is significant evidence of a local nonzero setting rather than a normal Postfix maximum. The authoritative parameter reference is Postfix Configuration Parameters.
The time window matters. Postfix’s anvil service uses anvil_rate_time_unit, whose documented default is 60 seconds. A limit of two requests in that window naturally looks like two messages per minute, even though the configuration is expressed as a count per anvil time unit. A remote provider’s throttling, queue retries, DNS delay or an application that opens a fresh connection for every message can create a similar visual pattern without any smtpd rate limit.
First diagnostic pass: find where the delay occurs
- Record the version and effective configuration. Run the following as a privileged user, adjusting the service name if your traffic arrives on another listener:
postconf mail_version postconf -n postconf -M submission/inetThe first command identifies the installed Postfix version. The second prints non-default settings from
main.cf. The third shows how the submission service is defined inmaster.cf. - Read the effective limit values.
postconf smtpd_client_message_rate_limit smtpd_client_connection_rate_limit smtpd_client_connection_count_limit smtpd_client_recipient_rate_limit smtpd_client_new_tls_session_rate_limit smtpd_client_auth_rate_limit smtpd_client_event_limit_exceptions anvil_rate_time_unitIf your version does not recognize
smtpd_client_auth_rate_limit, omit it; that control is available only in newer versions.#1 Best Overall
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- Search the mail log for the affected sender. Filter by the client IP, authenticated username, queue ID and the listener name. Look for explicit reject or defer text, references to
anvil, and the SMTP command involved: connection, AUTH,MAIL FROM,RCPT TOor message submission. A logged refusal or temporary deferral at the SMTP stage is evidence of a client-side limit. A successful acceptance followed by a queue ID is not. - Compare configuration layers. A value in
main.cfcan be replaced for one service with an-ooption inmaster.cf. Check the definitions forsubmission,smtpsand any custom listener that receives the application’s connections. - Inspect queue state.
postqueue -pRecord the number of queued messages, the age of the oldest entries and representative queue IDs. Then correlate those IDs with the log to see whether Postfix accepted the messages immediately and is waiting for downstream delivery.
Choose the correct troubleshooting branch
| Observed behavior | Likely location of the delay | Evidence to confirm |
|---|---|---|
| Connections, AUTH, MAIL FROM, RCPT TO or submission requests are rejected or deferred at a regular cadence | Inbound smtpd/anvil control | Log line names the client, service and limit-related rejection or deferral |
| SMTP returns success and assigns a queue ID, but delivery happens later | Queue scheduling, transport, DNS, TLS or recipient host | Acceptance timestamp is prompt; later log entries show retries or remote responses |
| Only one IP or account sees the pattern | Per-client setting, account policy or application behavior | Other senders on the same Postfix instance are faster |
| Every sender is slow | Global transport, queue, DNS, resource or remote-delivery issue | Queue age and delivery responses rise across unrelated clients |
| The interval is highly regular at roughly 60 seconds | Configured count combined with anvil’s time unit | Nonzero rate parameter and anvil_rate_time_unit value match the cadence |
| The interval varies with recipient domain or remote response | Downstream throttling or retry/backoff | Logs contain remote 4xx responses, connection timeouts, TLS errors or DNS failures |
Fix an inbound Postfix rate limit
Change the specific control implicated by the log, not every limit at once. The Postfix documentation states: “The purpose of this feature is to limit abuse. It must not be used to regulate legitimate mail traffic.” Keep protection for unknown clients and scope any exception to the trusted sender that needs it.
Message-delivery request limit
smtpd_client_message_rate_limit limits the number of message delivery requests a client may make in the interval specified by anvil_rate_time_unit. Its documented default is 0, meaning no limit. If the log shows this control and the sender is legitimate, set a value appropriate to the application or set it to 0 only when you have decided that this protection is unnecessary for that service.
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Connection and simultaneous-connection limits
smtpd_client_connection_rate_limit limits new connections in the anvil time window, while smtpd_client_connection_count_limit limits simultaneous connections from one client. An application that opens and closes a connection for every message can hit a connection limit even when its message count is modest. If the logs identify one of these controls, tune that parameter or change the application to reuse a submission connection.
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postconf -e 'smtpd_client_connection_rate_limit=0'
postconf -e 'smtpd_client_connection_count_limit=20'The second value is an example of a scoped operational choice, not a universal recommendation. Choose a concurrency level your CPU, memory, disk and downstream transport can sustain.
Recipient rate limit
smtpd_client_recipient_rate_limit counts recipients submitted by a client in the configured interval. A sender that puts many recipients into each message can reach this limit even when it sends only a few messages. Raise or remove this control only after confirming that recipient commands, rather than message requests, are being rejected.
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smtpd_client_new_tls_session_rate_limit can affect clients that repeatedly negotiate TLS. In newer Postfix versions, smtpd_client_auth_rate_limit can limit AUTH commands. A client that retries authentication or starts a new TLS session for each message may therefore appear to be limited to a small message rate. Fix the retry loop or connection reuse first; then adjust the identified parameter if the policy is intentional.
Trusted-client exceptions
smtpd_client_event_limit_exceptions identifies clients exempt from these event limits. Its documented default is $mynetworks. Review the actual mynetworks definition before adding anything. A broad subnet exception can let compromised hosts relay or consume resources, so prefer a narrowly defined submission network or authenticated service rather than an entire address range.
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postconf mynetworks
postconf smtpd_client_event_limit_exceptionsIf the sender is trusted but is not in the intended exception scope, correct the scope rather than disabling every safeguard. If a limit is not needed on a dedicated, controlled listener, set that particular parameter to 0 according to the documentation for your installed version.
Why anvil can look like a per-minute throttle
Anvil maintains the counters used by smtpd’s client event controls. The documented default for anvil_rate_time_unit is 60 seconds. For example, a message limit of two with that time unit permits two requests in each 60-second interval, producing the symptom in the title. The exact cadence can differ at interval boundaries and when several processes observe the same client.
On low-traffic systems, anvil processes pick up main.cf changes as they run. A postfix reload can speed adoption on systems where old processes would otherwise keep the previous value longer. Do not treat anvil as the cause solely because the word appears in a log; match the event to the configured control and the SMTP command that was delayed.
Check for a service-specific master.cf override
Many installations use a stricter policy on the public SMTP listener and a different policy on authenticated submission. A line such as -o smtpd_client_message_rate_limit=2 under submission/inet overrides the global value for that service. Conversely, changing main.cf will not remove an override that remains in master.cf.
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- Display the service definition:
postconf -M submission/inet - Open
master.cfand inspect continuation lines beginning with-ounder the listener that accepted the connection. - Compare each overridden parameter with the global value shown by
postconf -n. - Remove or edit only the override that matches the logged event, preserving stricter limits on untrusted listeners.
Always identify the receiving service from the log before changing a value. A client using port 587 may not be governed by the same settings as a client using port 25 or an alternate internal listener.
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When the SMTP client receives a successful response and Postfix assigns a queue ID, an smtpd rate limit has already allowed that transaction. Investigate the delivery path instead.
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Use postqueue -p to identify growing queues and old messages. Follow a representative queue ID in the mail log. Look for the destination host, connection attempts, DNS lookups, TLS negotiation, remote status codes and retry times. A remote 4xx response, timeout or TLS failure explains delayed delivery without any inbound client throttle.
Check transport concurrency and backoff
Postfix schedules outbound deliveries according to transport concurrency and retry rules. A destination that temporarily defers mail can cause repeated attempts with increasing intervals. If only one recipient domain is affected, compare its responses with successful domains before changing global limits. Increasing concurrency against a provider that is already deferring can make the queue worse.
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Verify DNS and TLS
Incorrect MX records, slow DNS resolution, certificate or protocol negotiation failures and blocked outbound connections all add time after acceptance. The relevant evidence is in the delivery log: identify whether Postfix is waiting for DNS, establishing a TCP connection, negotiating TLS or receiving a remote SMTP reply.
Inspect application submission behavior
An application that opens a new connection for every small message can spend most of its time on TCP, TLS and AUTH setup. Postfix tuning guidance recommends submitting multiple recipients in one message when privacy and message semantics allow it, and using SMTP submission rather than invoking /usr/sbin/sendmail repeatedly. Reusing an authenticated SMTP connection can remove client-side overhead without weakening Postfix abuse controls.
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Apply changes safely and verify the result
- Capture a baseline. Save the effective parameters, queue size, oldest queue age, relevant log lines and a timed test from a controlled sender. Record the client IP, account, listener and destination domains.
- Change one implicated setting. Keep the sender scope and time window explicit. Do not simultaneously raise message, connection, recipient and TLS limits, because you will not know which change affected the result.
- Validate syntax and effective values.
postfix check postconf smtpd_client_message_rate_limit smtpd_client_connection_rate_limit smtpd_client_connection_count_limit smtpd_client_recipient_rate_limit smtpd_client_new_tls_session_rate_limit smtpd_client_event_limit_exceptions anvil_rate_time_unit - Reload Postfix.
postfix reloadAllow the relevant service and anvil processes to adopt the change.
- Repeat the same controlled test. Use the same sender, listener, message size and recipient pattern. Compare SMTP acceptance timestamps, queue age and remote-delivery responses with the baseline.
- Watch for abuse signals. A faster trusted client is not proof that the setting is safe for anonymous traffic. Review authentication failures, connection volume, queue growth and outbound reputation after the change.
Common failure modes and their fixes
Setting the global value to zero changes nothing
This usually means the active listener has a master.cf -o override, the edited file is not the one used by the running instance, or the observed delay is downstream. Recheck the service definition and the log’s listener name.
The sender is exempt but still slow
Being in $mynetworks bypasses event limits; it does not bypass remote-provider throttling, DNS failures, queue scheduling or an application’s connection setup. Confirm that the SMTP transaction was accepted and inspect the queue and delivery log.
Raising a limit increases abuse
Rate controls exist to limit abusive clients. Keep conservative values on public listeners, use authenticated submission for known applications and create narrow exceptions instead of allowing broad networks. Monitor queue growth and authentication logs after every change.
The apparent rate changes when the batch size changes
This points toward recipient counting or application behavior rather than a simple message-request limit. Compare the number of SMTP transactions, recipients per transaction, connections and AUTH attempts in the logs.
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Practical decision checklist
- Is there an explicit reject or defer at the SMTP stage? If yes, identify the named parameter and listener.
- Is the relevant parameter nonzero when queried with
postconf? - Does
master.cfoverride the global setting for the active service? - Does the cadence match
anvil_rate_time_unit, normally 60 seconds? - Is the affected scope one IP, account or listener, or does it affect every sender?
- Did Postfix accept the message and assign a queue ID before the delay?
- Do queue logs show DNS, TLS, timeout or remote 4xx evidence?
- Can the application reuse SMTP connections or batch recipients without changing message privacy or semantics?
- After the change, did
postfix check,postconfand a controlled retest confirm the intended result?
Official references
Use the current postconf(5) parameter reference for the exact defaults and version-specific controls. The Postfix tuning guide describes the message-request limit as “The maximum number of message delivery requests that an SMTP client may make in the time interval specified with anvil_rate_time_unit (default: 60s).” Its source is TUNING_README.
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