Objective
Diagnose an SMTP rejection from the exact response code by identifying the rejecting server and checking 4xx/5xx semantics, authentication, TLS, sender, recipient, DNS, SPF, DKIM, DMARC, reputation, and a minimal reproducible test.
Prerequisites
- The sending domain, sender address, recipient and the exact mail path or provider.
- Access to DNS and mail-system logs or message tracing appropriate to the environment.
- Administrative access appropriate to the system being changed or diagnosed.
- A clearly identified scope: affected users, systems, addresses, services and the time of the observed problem.
- A maintenance or test window when the procedure can affect production traffic or availability.
- A copy of the current configuration or other recovery material before any irreversible action.
Step-by-step procedure
Establish the baseline and scope
Before changing anything, reproduce the issue or document the requested change on a representative system. Record the affected users or services, exact time, current configuration, recent changes and a known-good comparison point. This baseline is the reference used to decide whether each later step improves the situation.
- The scope and current state are documented well enough to reproduce or verify the procedure.
The rejecting server
Examine this diagnostic area: the rejecting server. Use the narrowest reproducible test, compare the observed state with the documented or expected state, record timestamps and exact errors, and correlate the result with adjacent dependencies. Do not change several settings at once: the purpose of this step is to establish whether this area is healthy, misconfigured, unavailable, or simply not involved in the failure.
- Evidence for this area is explicit, reproducible and consistent with the intended design.
4xx/5xx semantics
Examine this diagnostic area: 4xx/5xx semantics. Use the narrowest reproducible test, compare the observed state with the documented or expected state, record timestamps and exact errors, and correlate the result with adjacent dependencies. Do not change several settings at once: the purpose of this step is to establish whether this area is healthy, misconfigured, unavailable, or simply not involved in the failure.
- Evidence for this area is explicit, reproducible and consistent with the intended design.
Authentication
Examine this diagnostic area: authentication. Use the narrowest reproducible test, compare the observed state with the documented or expected state, record timestamps and exact errors, and correlate the result with adjacent dependencies. Do not change several settings at once: the purpose of this step is to establish whether this area is healthy, misconfigured, unavailable, or simply not involved in the failure.
- Evidence for this area is explicit, reproducible and consistent with the intended design.
TLS
Examine this diagnostic area: TLS. Use the narrowest reproducible test, compare the observed state with the documented or expected state, record timestamps and exact errors, and correlate the result with adjacent dependencies. Do not change several settings at once: the purpose of this step is to establish whether this area is healthy, misconfigured, unavailable, or simply not involved in the failure.
- Evidence for this area is explicit, reproducible and consistent with the intended design.
Sender
Examine this diagnostic area: sender. Use the narrowest reproducible test, compare the observed state with the documented or expected state, record timestamps and exact errors, and correlate the result with adjacent dependencies. Do not change several settings at once: the purpose of this step is to establish whether this area is healthy, misconfigured, unavailable, or simply not involved in the failure.
- Evidence for this area is explicit, reproducible and consistent with the intended design.
Recipient
Examine this diagnostic area: recipient. Use the narrowest reproducible test, compare the observed state with the documented or expected state, record timestamps and exact errors, and correlate the result with adjacent dependencies. Do not change several settings at once: the purpose of this step is to establish whether this area is healthy, misconfigured, unavailable, or simply not involved in the failure.
- Evidence for this area is explicit, reproducible and consistent with the intended design.
DNS
Examine this diagnostic area: DNS. Use the narrowest reproducible test, compare the observed state with the documented or expected state, record timestamps and exact errors, and correlate the result with adjacent dependencies. Do not change several settings at once: the purpose of this step is to establish whether this area is healthy, misconfigured, unavailable, or simply not involved in the failure.
- Evidence for this area is explicit, reproducible and consistent with the intended design.
SPF
Examine this diagnostic area: SPF. Use the narrowest reproducible test, compare the observed state with the documented or expected state, record timestamps and exact errors, and correlate the result with adjacent dependencies. Do not change several settings at once: the purpose of this step is to establish whether this area is healthy, misconfigured, unavailable, or simply not involved in the failure.
- Evidence for this area is explicit, reproducible and consistent with the intended design.
Validate the complete service
Repeat the original user, system or application workflow from the real source and verify the complete result, not only one command or one local check. Confirm that logs and monitoring show the expected behavior and that no temporary debug, bypass, test account, rule or maintenance setting remains enabled.
- The end-to-end service works or the remaining failure is isolated to a clearly identified component.
Technical commands from the original procedure
These technical blocks are preserved byte-for-byte from the historical procedure and kept in their original order. Review names, addresses, paths and parameters before use.
Resolve-DnsName <SMTP-HOST>Test-NetConnection <SMTP-HOST> -Port <PORT>openssl s_client -starttls smtp -connect <SMTP-HOST>:587 -servername <SMTP-HOST>Resolve-DnsName -Type MX <DOMAINE>Resolve-DnsName -Type TXT <DOMAINE>Resolve-DnsName -Type TXT _dmarc.<DOMAINE>Validation
The procedure is validated when:
- The original symptom or change request has been tested end to end.
- The effective configuration matches the intended design and no unexplained error remains in the relevant logs.
- Temporary troubleshooting controls have been removed and monitoring remains normal.
- The result, evidence and any follow-up action are documented.
Rollback
- Restore the configuration, policy, binding, route, credential assignment or service state recorded in the baseline when the change does not meet its success criteria.
- Remove temporary rules, test objects and diagnostic settings that were introduced only for the procedure.
- After rollback, repeat the minimum health checks to confirm that the previous service level has been restored.
Troubleshooting / common errors
- Always distinguish envelope MAIL FROM, visible From, Reply-To and authenticated identity.
- DNS changes can be correct at the authoritative server while recursive resolvers still hold the previous value until TTL expiry.
- If the result changes between tests, compare source, destination, identity, time and policy context before changing additional settings.
- If a command succeeds but the application still fails, continue at the next protocol or application layer instead of widening access.
- If the expected evidence is missing, verify that logging, auditing and the test path actually cover the failing component.
- If the change does not improve the measured symptom, restore the previous state and reassess the working hypothesis.