Start with one test message and the recipient’s exact response. A bounce, SMTP transcript, or delivered message’s headers can show whether the failure is in your MTA, domain authentication or DNS, the sending IP and network, or the recipient’s policy. Fix the fault that the evidence identifies; no configuration can guarantee inbox placement because each provider makes its own filtering decisions.
What kind of delivery failure are you seeing?
Classify the symptom before changing DNS or mail-server settings. A permanent rejection, a temporary deferral, a message delivered to spam, and a message that seems to disappear point to different parts of the delivery path.
- Permanent rejection: The recipient server refused the message. Keep the complete SMTP response, including any enhanced status text; the reason may be a policy, authentication, identity, or message-format issue.
- Temporary deferral: The recipient did not accept the message at that time. The response code and text are more useful than the fact that the message is still queued. Avoid increasing sending volume or repeatedly forcing retries while the cause is unresolved.
- Spam-folder placement: The recipient accepted the message, but its filtering system classified it as spam. Authentication, sender reputation, message characteristics, and recipient feedback can all matter.
- No visible delivery or bounce: Check your server’s logs and queue first, then look for evidence from the recipient system. A message that has left your server is not necessarily in the recipient’s inbox.
Use a test recipient you control where possible. Record the time, recipient provider, sending IP, complete bounce or SMTP response, and full headers from a delivered test. Redact addresses, message contents, and credentials before sharing logs or transcripts.
How do you trace one test message through Postfix?
Begin at the sending server, not by making several DNS changes at once. The Postfix Project’s Debugging Howto says the first order of business when Postfix does not receive or deliver mail is to look for errors that prevent it from working properly.
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- Find the earliest relevant log entry. Search the mail log around the test’s timestamp and inspect the first warning, error, fatal, or panic message associated with it. Later errors may be consequences of the first fault.
- Check the queue. Determine whether the message is still waiting, repeatedly deferred, or has been handed off. A queued message and a message accepted by a remote server call for different next checks.
- Read the remote server’s response. Preserve the complete response rather than relying on a short summary such as “rejected” or “deferred.” The enhanced status text can identify a provider-specific policy or identity problem.
- Capture a session only when needed. If the logs suggest a connection or SMTP protocol problem, a session trace can help locate the failure. Use a test message and redact sensitive data before sharing the trace.
Postfix log locations depend on the system’s logging configuration, so use the location configured for your server rather than assuming a particular file.
Are SPF, DKIM, DMARC, and From alignment working?
For a delivered test, inspect the Authentication-Results header added by the receiving system. Check the reported SPF and DKIM results, then check DMARC and whether the authenticated domain aligns with the visible From: domain. These are related checks, not interchangeable passes: success for one mechanism does not prove that all the others pass or that alignment is correct.
- SPF: Confirm the SPF record covers every legitimate system that sends for the domain, including web applications and relays. Keep it current and publish only one SPF record for a given name.
- DKIM: Check whether the receiving system reports a DKIM pass for the test message. If a relay is involved, verify that its signing configuration is appropriate for your domain.
- DMARC and alignment: Check the DMARC result and the relationship between the visible From domain and the domains authenticated by SPF or DKIM. For Gmail bulk senders, Google requires alignment of the visible From domain with SPF or DKIM.
Google’s Gmail guidance says all senders need SPF or DKIM. Senders sending more than 5,000 messages per day to personal Gmail accounts fall under Google’s bulk-sender requirements, which call for SPF, DKIM, and DMARC. Google recommends setting up all three even when a sender is below that threshold. These are Gmail-specific requirements, not a statement of the rules used by every mailbox provider.
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After an SPF record change, Google Workspace Admin Help says SPF authentication can take up to 48 hours to start working. Check the receiving system’s result rather than assuming the edit took effect immediately.
Does the sending IP have matching forward and reverse DNS?
Verify the reverse DNS (PTR) for the outbound IP, then confirm that the hostname in the PTR record resolves forward through A or AAAA records to that same IP. Check the hostname your SMTP server uses as well as the public identity of the connection. If the server sends over both IPv4 and IPv6, check each address’s reverse and forward identity separately.
Google explicitly requires valid forward and reverse DNS in its Gmail sender guidance. A mismatch can be enough to trigger a provider-specific rejection even when SPF or DKIM passes. For example, Gmail’s 4.7.23 response is associated with a missing or mismatched PTR; treat that code as Gmail-specific rather than a universal interpretation of every provider’s errors.
Is the connection or message itself failing?
Use the log and SMTP response to distinguish a local delivery fault from a problem in the connection to the recipient. Confirm that outbound SMTP connections can be made and that TLS is used as appropriate. Review the SMTP exchange for a failed connection or protocol error, and check that the message conforms to RFC 5322 formatting.
If the recipient server accepts the message but it lands in spam, a clean SMTP exchange does not establish that the message is wanted or will reach the inbox. If the connection fails before the recipient evaluates the message, investigate the network path and server configuration before changing content or authentication records.
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Interpret the exact SMTP code together with the enhanced status text and the provider that returned it. A 4xx response generally indicates a temporary condition, while a 5xx response generally indicates a rejection; the accompanying text is needed to identify the actual issue. Do not assume all responses in either class have the same cause.
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For Gmail recipients, Google Postmaster Tools can provide information about authentication, reputation, spam feedback, and delivery errors when data is available. Use it alongside a specific test message’s headers and SMTP response, not as a replacement for them. Google’s 4.7.32 response, for example, relates to From alignment; it is a Gmail-specific clue.
Google’s Gmail sender guidance says to keep the reported spam rate below 0.3%; it recommends staying below 0.10% and avoiding 0.30% or higher. These are Google’s published Gmail figures, not general industry-wide thresholds. A low reported rate does not guarantee inbox placement.
Should you change how or how much you send?
For opted-in bulk or subscription mail, monitor recipient complaints, honor unsubscribe requests, and avoid abrupt volume spikes. Google’s Gmail bulk-sender guidance also requires one-click unsubscribe for marketing or subscribed messages. If deferrals or bounces begin, reduce sending while you investigate rather than pushing more mail into a temporary failure.
Review whether the recipients expect the mail and whether your sending pattern has changed. Authentication can establish aspects of sender identity, but it does not make unwanted mail welcome or determine how a recipient provider weighs complaints and reputation.
Independent reader supportYour contribution helps us test, update, and keep practical guides available for everyone.Should you send directly or use an outbound SMTP relay?
Direct delivery gives you control over the sending IP and the server logs, but the IP and hosting network must be accepted by recipient providers. A relay can be a practical alternative if your host or ISP does not permit reliable direct SMTP or your IP has poor acceptance. It changes the sending path; it does not eliminate responsibility for domain authentication or sending practices.
| Factor | Direct to recipient MX | Outbound SMTP relay |
|---|---|---|
| Sending IP and logs | You control the sending IP and inspect your own server logs. | The relay controls the outbound IP; the relay’s diagnostic detail depends on its service. |
| Network acceptance | Acceptance depends on your host or ISP network and the reputation of your sending IP. | Can change the sending path when your own network or IP is a constraint; acceptance is not guaranteed. |
| Authentication and alignment | You configure your sending domain’s SPF, DKIM, and DMARC and check visible From alignment. | You still need correct domain authentication and alignment; include the relay in SPF where appropriate and verify how it signs. |
| Operational burden | You operate and diagnose the direct sending path. | Some sending infrastructure is delegated, but your domain configuration and message practices still need attention. |
| Third-party dependence | No outbound relay is involved. | Delivery depends in part on a third party’s service and the diagnostic information it provides. |
Before routing mail through a relay, verify that it supports the signing and authentication your domain needs, is represented correctly in SPF, preserves the required alignment, and provides enough detail to troubleshoot failures. A relay cannot repair a bad domain policy, missing authentication, or unwanted mail practices.
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