The Tool Desk
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A local success response does not prove Internet delivery. It proves only that Exim accepted the message for processing. The decisive evidence is the later delivery record and its remote response.
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Start with a controlled diagnosis
Do not begin by changing routers, disabling TLS, or permitting broader relaying. First choose one message that failed and record its Exim message ID. Follow that ID through the main log, reject log, panic log, and any per-message log available on the server. Exim writes a delivery line for each delivery attempt for each address, so the same ID lets you distinguish submission, queueing, lookup, connection, and recipient-policy failures.
- Choose one reproducible message. Send a test to a local mailbox and, separately, to an external domain if possible. Note the sender, recipient, submission time, and message ID returned by the submission path.
- Search every relevant log for that ID. Collect the initial acceptance line, queue or retry records, transport attempts, remote SMTP replies, and any frozen-message notice.
- Classify the result before changing configuration. A successful delivery, a temporary defer, a permanent failure, and a frozen message require different actions.
- Retry once under observation. If the message is deferred, perform a controlled queue run and watch the logs for the new attempt. The new line normally contains the most useful error.
Exim’s official log documentation describes delivery lines, deferrals, frozen messages, SMTP connection failures, host-lookup failures, and retry records. Treat those records as the primary evidence; a mail client’s generic “sent” notification is not a delivery result.
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Understand where “sending” can stop
An Exim message passes through several independent stages. Knowing the stage prevents a fix for one problem from making another problem worse.
| Layer | What must work | Typical evidence | Safe first action |
|---|---|---|---|
| Submission | A local process or authenticated SMTP client can hand the message to Exim. | No message ID, client rejection, or an SMTP/ACL error at submission time. | Check the daemon state, listening interface, port, sender policy, and credentials. |
| Queue | The message is stored and a queue runner is allowed to process it. | Message exists in the spool but has no recent delivery attempt, or the log says retry time has not arrived. | Check queue-only settings, queue-run behavior, spool locks, and the recorded retry time. |
| DNS and routing | Exim can resolve the recipient domain and obtain a usable destination. | Host-lookup failure, no usable MX or A result, or a router that cannot select a transport. | Verify the recipient domain’s DNS from the server and inspect the router result. |
| Network | The server can establish outbound SMTP connectivity. | Connection refused, timeout, unreachable host, or an egress policy error. | Check firewall rules, provider restrictions, and TCP port 25 reachability. |
| SMTP, TLS, and authentication | The remote server accepts the session, encryption, credentials, and envelope. | TLS negotiation failure, authentication failure, unexpected disconnect, or a remote 4xx/5xx reply. | Use the exact response to correct the relevant transport or credential setting. |
| Recipient policy | The destination accepts the sender, address, identity, and message. | Permanent rejection, policy block, invalid recipient, or a bounce. | Fix the address or identity being rejected; do not grant open relay access. |
Step 1: Find the message and read Exim’s logs
Use the message ID as your trace key
Search for the ID rather than only the recipient address. A single message can produce multiple lines as it moves from acceptance to routing, retry scheduling, and final delivery. Correlating the ID prevents you from mixing a new test with an older deferred message or another recipient on the same submission.
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Read the logs in chronological order. The first line establishes whether Exim accepted the message. Later lines show which router and transport were selected, whether a host lookup succeeded, whether a connection was attempted, and what the remote server returned. A panic-log entry points to an internal or configuration problem that should be resolved before repeated queue runs.
What to collect for one failure
- The message ID and envelope sender.
- Every recipient address, because one message can succeed for one address and defer or fail for another.
- The selected router and transport, if logged.
- The complete host-lookup, connection, TLS, authentication, and remote SMTP response.
- The retry time, defer reason, or frozen status.
- Whether the error is marked temporary or permanent.
Do not replace a complete response with a paraphrase such as “Gmail blocked it” or “DNS is broken.” The exact SMTP code and text determine whether Exim should retry, bounce, or wait.
Step 2: Confirm Exim is running and accepting submissions
Separate a daemon problem from a delivery problem. Exim can be configured as a listening daemon or invoked through an inetd-style service, and locally submitted messages follow a different path from incoming SMTP connections. The Exim command-line documentation identifies daemon mode with -bd; the SMTP-processing documentation explains how a listening service handles connections.
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Check the daemon state and listener
- Confirm that the Exim service is running under your Linux service manager. If it is stopped or repeatedly restarting, inspect the service error output and Exim’s panic log before attempting delivery.
- Confirm that Exim is listening on the interface and SMTP port intended for your installation. A daemon bound only to loopback will accept local submissions but not connections from an application on another host.
- Verify that the client is using the same host, port, encryption mode, and authentication method configured in Exim. A client pointed at the wrong port can fail before Exim sees the message.
- Submit a small test and verify that Exim assigns a message ID. If no ID is created, continue troubleshooting submission rather than the queue.
By default, Exim listens for incoming SMTP connections on the standard SMTP port on all of the host’s running interfaces. That is a default, not a guarantee: local configuration, a service wrapper, a firewall, or a provider policy can change the effective behavior.
Distinguish local submission from SMTP reception
A local web application may submit through a local socket or command-line interface while an external mail client connects over SMTP. Success in the first path says nothing about the second. Test each path that your users actually need, and record which one creates the message.
Step 3: Determine whether mail is queued, deferred, or locked
Queueing is normal when Exim cannot complete delivery immediately. It is not evidence that delivery succeeded. Queue-only operation, a queue runner that is not active, a locked spool file, or a temporary remote error can leave a message waiting even though local submission works.
Read the queue state in context
- Queued with no attempt: Check whether the installation is configured to accept messages without immediate delivery and whether a queue runner is scheduled.
- Deferred with a retry time: Exim is following its retry policy. “Retry time not reached” means the next scheduled attempt has not arrived; it does not identify the original cause.
- Repeated attempts with the same error: Fix the underlying DNS, network, TLS, authentication, or recipient issue instead of forcing endless retries.
- Spool or lock indication: Determine which process owns the lock and whether a stale process or filesystem problem is preventing the queue runner from working.
Run one observed queue attempt
Exim’s command-line chapter documents queue-run behavior with -q. Use the queue-run command appropriate to your deployment while watching the main and per-message logs. The goal is to capture one fresh transport result, not to flood a remote server with repeated attempts. If the log immediately reports that retry time has not arrived, note the scheduled time and inspect the previous attempt’s error; changing the clock or repeatedly invoking the queue does not repair the original failure.
Exim’s SMTP transport documentation explains that when none of the hosts for an address can be delivered to and the errors are not permanent rejections, the address is placed on a separate transport queue. That behavior is protective: temporary failures are retried according to policy rather than converted into an immediate bounce.
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Step 4: Match the exact transport error to the right fix
| Log symptom | What it means | What to check next |
|---|---|---|
| Host lookup failure or no usable MX/A result | Exim cannot obtain a destination for the recipient domain. | Query the domain’s DNS from the server, check resolver configuration, and verify that the domain publishes a usable mail route. |
| Connection refused | The destination host or an intervening device actively rejected the TCP connection. | Confirm the selected destination, outbound firewall rules, and whether the remote service accepts SMTP from your network. |
| Connection timeout | No response arrived before the network or transport timeout. | Check routing, egress filtering, provider restrictions on TCP port 25, and the remote host’s reachability. |
| TLS negotiation failure | The two SMTP peers could not agree on or complete encryption. | Read the detailed TLS error, verify the configured certificate and protocol expectations, and compare the transport’s TLS mode with the destination’s requirements. |
| Authentication failure | The remote submission service rejected the supplied credentials or authentication exchange. | Verify the account, secret, authentication mechanism, and whether the transport is using the intended submission host and port. |
| Unexpected remote disconnect | The peer closed the connection before completing the SMTP exchange. | Use the surrounding log lines to determine whether the disconnect occurred during greeting, TLS, authentication, envelope, or message data. |
| Remote 4xx response | A temporary condition caused the destination to defer acceptance. | Leave retry policy intact, correct any local condition indicated by the response, and monitor the next scheduled attempt. |
| Remote 5xx response | A permanent rejection or policy decision was returned. | Correct the address, sender identity, authentication, or policy issue named by the response; a permanent failure normally becomes a bounce. |
Exim supports fallback hosts and authentication-related transport controls, but those options should be used only when the log shows that the selected route or authentication path is the problem. Adding a fallback that points to an unrelated host can hide a DNS error and create a new policy or security problem.
Step 5: Check DNS, outbound access, and server identity
Verify recipient-domain DNS from the mail server
A domain can resolve on your workstation and fail on the server because the server uses a different resolver, firewall, or network path. Perform the lookup from the Linux host running Exim. Confirm that the recipient domain has a usable MX result; where the routing rules permit it, an A result may also be relevant. Record whether the failure is a resolver timeout, an authoritative negative answer, or simply the absence of a usable mail host.
Do not “fix” a lookup failure by hard-coding a destination until you understand the cause. Changing the route can send mail to the wrong host and bypass the recipient domain’s published policy.
Check TCP port 25 egress
Many hosting providers and ISPs restrict outbound TCP port 25 to limit abuse. Debian’s Exim FAQ specifically calls out ISP port-25 blocking as a deployment issue. If DNS succeeds but every external destination times out or is refused, ask the provider whether port-25 egress is blocked and whether it can be enabled. A local Exim configuration change cannot remove an upstream block.
Also inspect the server’s own outbound firewall and any cloud security-group or network ACL rules. Compare a failing external destination with a known reachable SMTP host only as a connectivity test; do not use a third-party relay unless you are authorized and have configured its authentication and policy requirements.
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Give the server a valid identity
The same Debian FAQ notes the need for an externally valid fully qualified domain name and system mail name. Ensure the hostname used by Exim is a real FQDN in your DNS and that the system mail name is not an unqualified local label. Where a recipient requires identity checks, verify forward DNS for that name and reverse DNS for the server address with the organization that controls the address space. A syntactically valid local hostname may still be rejected by a remote policy.
Step 6: Resolve relay refusals without opening an open relay
“Relay not permitted” is a policy result, not a generic delivery error. It means the sender, source host, destination, or authentication state did not match the ACL or router rules that authorize relay for that path.
Identify which submission path was rejected
- Determine whether the sender is local, authenticated, or coming from a trusted internal network.
- Check whether the destination is local to the server or an external domain. A server may accept local delivery while refusing unauthenticated forwarding.
- Confirm that the client authenticated before issuing the envelope commands, and that it used the credentials and mechanism expected by the configured ACL.
- Read the reject log for the specific condition. The wording often distinguishes an unauthenticated client from an unauthorized sender or destination.
Correct the intended submission path and credentials narrowly. Do not add every source network to a trusted list, disable recipient checks, or permit unauthenticated relay simply to make one test pass. A broad relay exception can turn the server into an abuse target and still fail at remote destinations.
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Exim’s delivery log notation tells you what happened to each address. Interpret the symbol together with the response text and message ID.
| Notation | Meaning | Operational response |
|---|---|---|
=> |
Successful delivery to that address or transport destination. | Check the recipient-side result if the user still cannot see the message; Exim has completed its handoff. |
== |
Temporary delivery defer. | Read the reason and retry time. Correct the underlying transient condition and allow the retry policy to operate. |
** |
Permanent failure or bounce. | Use the remote response to correct the address, routing, identity, or policy condition before resubmitting. |
| Frozen | Exim has stopped normal automatic processing for the message after a recorded delivery problem. | Inspect the freeze reason and the complete message log. Unfreezing without fixing that reason simply repeats the failure. |
A message can have different outcomes for different recipients. Do not treat one successful => line as proof that every address on the message was delivered.
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Common symptom branches
Local mail works, but Gmail or other external domains do not
Start at the external-delivery boundary: inspect the recipient-domain lookup, the selected MX host, and the outbound connection result. If the log shows timeouts or refusals for many unrelated domains, investigate port-25 egress and the server firewall. If connections succeed but the remote host rejects the session, examine the FQDN, forward and reverse DNS, TLS, sender identity, and the exact policy response. A local 250 acceptance confirms only that Exim accepted the message for queueing.
Messages remain queued and say “retry time not reached”
Find the preceding attempt for the same message ID. The retry notice is a scheduling statement, not the root cause. Read the earlier DNS, connection, TLS, authentication, or remote SMTP error, then verify that a queue runner is active. A queue-only configuration or a missing queue run can produce waiting messages even when Exim is healthy enough to accept new submissions.
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Every submission fails immediately
Check whether Exim is running and listening on the interface and port used by the client. If a message ID is never created, focus on the client-to-Exim connection, sender ACL, and authentication. If an ID is created and an immediate reject follows, use the reject log to correct the policy condition rather than changing remote-delivery transports.
Only one recipient domain fails
Compare that domain’s DNS result and the remote response with a domain that succeeds. A single-domain failure commonly points to that domain’s MX availability, a recipient address or policy rejection, or a destination-specific TLS requirement. Keep the scope narrow; do not alter global relay or routing rules when the evidence names one destination.
The queue stops moving after a configuration change
Inspect the panic log and the first delivery attempt after the change. A syntax or transport error can prevent normal processing, while a spool lock can prevent a queue run. Restore the last known-good setting only after preserving the error details, then validate one test message before releasing the broader queue.
A safe recovery checklist
- Record one message ID, sender, recipients, and submission time.
- Read the main, reject, panic, and per-message logs for that ID.
- Confirm whether Exim is listening and whether the client is using the intended interface and port.
- Determine whether the message is queued, deferred, permanently failed, or frozen.
- Run one observed queue attempt with
exim -qwhen appropriate for the installation. - Apply the smallest fix that matches the exact error: DNS, network, identity, TLS, authentication, or recipient policy.
- Test a local recipient and an external recipient separately.
- Verify the resulting log symbol and remote response before processing a larger backlog.
When to stop retrying and escalate
Escalate to the hosting provider or network operator when port-25 egress is blocked, reverse DNS is controlled outside your account, or the remote network is unreachable despite correct local routing. Escalate to the recipient’s mail administrator when a specific destination returns a persistent policy rejection and your DNS, identity, TLS, and authentication records are correct.
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