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A free scan shows the junk files, broken settings and background clutter dragging Windows down - then fixes them in one click.Free scan · Windows 10 & 11Postfix mail usually lands in spam because the receiving provider cannot verify the sender or does not yet trust the sending IP and domain. Fix authentication and DNS first—SPF, DKIM, DMARC, reverse DNS (PTR), and trusted TLS—then investigate complaints, blocklists, and delivery rate. Changes to main.cf alone rarely solve a reputation problem.
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Start by sending a message to a Gmail mailbox and opening its Authentication-Results header. It tells you whether SPF, DKIM, and DMARC passed and whether they aligned with the domain visible in the From: header. A pass on all three gives you a reliable baseline before you change anything else.
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Spam placement, rejection, and throttling are different problems
“Spam” means the message was accepted and placed in a junk folder. A rejection means the receiving server refused it, commonly with a 5xx SMTP response. A deferral means temporary refusal, usually a 4xx response; Postfix queues the message and retries. Treating all three as “Postfix spam” leads to the wrong fix.
| Symptom | What it means | First evidence to collect |
|---|---|---|
| Message appears in Gmail Spam | Gmail accepted the message but classified it as unwanted. | Authentication-Results, From-domain alignment, and complaint/reputation data. |
| SMTP 550, 553, or 554 rejection | The recipient made a permanent policy or address decision. | Complete SMTP status and enhanced status code in the Postfix log. |
| SMTP 421 or another 4xx deferral | The recipient is asking you to slow down or try again later. | Queue state, retry history, destination response, and delivery rate. |
| Messages remain in the Postfix queue | Delivery is delayed; this is not proof of spam classification. | postqueue -p and the corresponding mail-log lines. |
Google’s help guidance distinguishes requirements for all senders from those for bulk senders: all senders need SPF or DKIM, while bulk senders need SPF, DKIM, and DMARC. Sender-volume definitions and enforcement details change, so check Google’s current sender documentation before relying on a quoted threshold.
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1. Read the receiving server’s authentication result
Do not begin by guessing at Postfix parameters. Send a controlled test from the same envelope sender, visible From domain, and outbound IP that you use in production.
- Send a message to a Gmail account that you control.
- Open the message menu and choose Show original.
- Find
Authentication-Resultsand record the SPF, DKIM, and DMARC result, the authenticated domain, and the selected DKIM selector. - Compare those domains with the visible
From:address. A technically valid signature can still fail DMARC alignment if it belongs to an unrelated domain. - Save the complete SMTP and Postfix log entry for the same message, including the queue ID.
Repeat the test after each DNS or signing change. DNS caches and queued messages can make a correction appear ineffective for a while; test a newly generated message rather than repeatedly releasing the same queue item.
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SPF authorizes the IP addresses and services allowed to send mail for a domain. Publish it as a TXT record at the domain used by your envelope sender (the SMTP MAIL FROM domain). It does not authenticate the visible From address by itself.
Use exactly one SPF TXT record
There must be one TXT record whose value begins with v=spf1. Two separate SPF records for the same domain produce a permanent SPF error. Combine mechanisms into one record instead of adding another record when a new service starts sending.
Stay within the DNS-lookup limit
An SPF evaluation allows at most 10 DNS lookups. The limit applies to mechanisms and modifiers such as include, a, mx, and redirect; nested includes count as they are evaluated. A policy can therefore fail even when every individual include is valid. Remove unused services, flatten only when you can maintain the resulting addresses safely, or assign different sending services to dedicated subdomains.
Authorize the actual Postfix path
List the public IPv4 and IPv6 addresses that really originate mail, and include any relay that handles final delivery. Do not authorize a private interface address. If your envelope sender is bounce.mail.example, publishing SPF only at example does not automatically cover the subdomain.
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3. Make DKIM signing and DNS agree
DKIM adds a cryptographic signature to each message. Postfix commonly passes mail to a DKIM signer such as OpenDKIM; the signer selects a private key and writes a DKIM-Signature header.
Check the selector character for character
If the signature contains s=selector1 and d=example.com, the public key must be available at selector1._domainkey.example.com. A selector mismatch is easy to miss because Postfix may continue delivering mail while receivers report dkim=fail or no usable signature.
Verify the complete key record
- Publish the public key as a TXT record at the exact selector name.
- Remove stray quotation marks or line-break characters introduced by a DNS editor.
- Confirm that the signer can read the private key and that its file permissions permit the signing service to open it.
- Use a 2048-bit key as the common choice, unless a receiving system imposes a different constraint.
- After rotation, leave the old selector published until messages signed with the old key have expired from queues and caches.
Inspect a fresh Gmail message and confirm that the DKIM result is pass, that the signing domain is the one you intend, and that the selector in the header matches the DNS record.
4. Add DMARC for alignment and reporting
DMARC evaluates whether SPF and/or DKIM both authenticate and align with the visible From domain. Alignment is the part that catches many otherwise “passing” configurations: SPF can pass for a bounce domain while DKIM passes for a different organizational domain, leaving DMARC unable to authenticate the From address.
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Publish a DMARC TXT record at _dmarc.example.com with a monitoring policy such as p=none, and provide a reporting address you control. A monitoring-only policy does not change delivery; it lets you inspect aggregate reports and discover senders you did not know were using the domain.
Move enforcement gradually
- Keep
p=nonewhile every legitimate stream is identified and alignment is corrected. - Review aggregate reports for unauthorized IPs, failing selectors, and subdomains with different behavior.
- Move to
p=quarantinewhen legitimate traffic consistently aligns. - Use
p=rejectonly after you are confident that important mail will not be rejected because of an overlooked relay or application.
DMARC policy is a domain-level instruction; it cannot repair a DKIM key, an SPF lookup overflow, or a bad PTR record. Fix those underlying issues separately.
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5. Do not confuse Google Workspace relay with self-hosted Postfix
A Google Workspace relay configuration and a server that delivers directly from Postfix have different authentication responsibilities.
| Sending design | SPF responsibility | DKIM responsibility | DMARC responsibility |
|---|---|---|---|
| Google Workspace handles outbound relay | Authorize Google’s sending service as directed by Workspace setup; the commonly used include is include:_spf.google.com. |
Enable domain signing in the Workspace Admin console and publish the supplied public key. | Publish and monitor a DMARC record for the visible From domain. |
| Self-hosted Postfix delivers directly | Authorize your own public sending IPs and any additional relays. | Operate your own signer and publish its selector’s public key. | Publish DMARC and ensure the signer or envelope domain aligns with From. |
Do not add Google’s SPF include merely because a mailbox is hosted there if your Postfix server is the only system sending mail. Conversely, do not omit a relay that actually submits your messages.
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6. Set reverse DNS (PTR) and a matching hostname
Receiving providers commonly check the reverse DNS of the connecting IP. The IP should have a PTR record that resolves to the hostname Postfix presents, and that hostname should resolve forward to the same IP. A missing or generic reverse name can reduce trust or trigger rejection even when SPF, DKIM, and DMARC pass.
- Choose a stable fully qualified hostname, such as
mail.example.com. - Set the server’s system hostname and Postfix
myhostnameto that name. - Ask the hosting or network provider that owns the IP address to set its PTR record to the same hostname. You normally cannot create this record in your ordinary domain DNS panel.
- Publish a forward A and, where applicable, AAAA record for the hostname pointing back to the sending address.
- Confirm the result from outside your network before testing delivery again.
If you change providers or addresses, repeat the PTR request. A correct PTR on an old IP does nothing for mail leaving a new address.
7. Deliver with trusted TLS
Use SMTP STARTTLS when Postfix connects to recipient servers and present a certificate issued by a publicly trusted certificate authority. A self-signed certificate can cause verification failures or downgrade trust. TLS encrypts the connection; it does not replace SPF, DKIM, DMARC, or reputation controls.
- Keep the certificate name consistent with the outbound hostname.
- Install the complete certificate chain, not only the leaf certificate.
- Renew before expiry and reload Postfix after renewal.
- Review logs for TLS negotiation failures and destinations that defer after a failed handshake.
Google expects encrypted delivery, but a successful TLS session alone does not guarantee inbox placement.
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8. Protect reputation and handle complaints
Once authentication is sound, recipient behavior becomes decisive. Send only to people who asked for the mail, remove hard bounces promptly, and stop sending to addresses that repeatedly produce complaints or no engagement. Keep transactional and marketing streams on separate subdomains or IPs when their audiences and sending patterns differ; a marketing complaint spike should not automatically damage password-reset traffic.
Monitor the spam-rate signal
Third-party guidance restating Google Postmaster Tools cites a 0.3% user-reported spam-rate ceiling. Treat that figure as a current-policy check, not a permanent guarantee: Google’s thresholds and measurement rules can change. Use Postmaster Tools data when it is available for your domain and confirm the current guidance before making operational decisions.
Make unsubscribing immediate
Marketing messages should include a one-click unsubscribe mechanism and process requests promptly. The cited guidance describes processing within two days. Suppressing an address immediately in your application is safer than waiting for a periodic list-cleaning job.
9. Check blocklists before changing Postfix
Look up every public IPv4 and IPv6 address that can send mail on widely used DNS blocklists. A listing does not prove that a message is spam, but it can explain a recipient’s permanent rejection. Record the list name, listing reason, and delisting instructions; do not repeatedly request removal without fixing the behavior that caused the listing.
Also check whether an old address, forgotten relay, or compromised web application is still sending. If only one IP is listed, route legitimate mail through a clean, correctly authenticated address while you investigate, but do not rotate addresses to evade a continuing abuse problem.
Independent reader supportYour contribution helps us test, update, and keep practical guides available for everyone.10. Diagnose outbound throttling in Postfix logs
A 421 response such as “unusual rate” indicates a delivery-rate problem, not necessarily spam-folder classification. The receiving domain is asking for slower or less concurrent delivery. Postfix will normally defer and retry, but an aggressive queue can keep triggering the same response.
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Use the queue and log together
postqueue -p
postconf -n
grep -E "status=(deferred|bounced|sent)" /var/log/mail.logUse the log location appropriate to your operating system. Search by queue ID, recipient domain, and the exact remote SMTP response. A 4xx response with repeated retries calls for rate reduction and scheduling changes; a 5xx response calls for policy, authentication, address, or reputation investigation.
Understand the anvil limit
Settings such as smtpd_client_connection_rate_limit = 30 govern the rate of inbound client connections to your Postfix server in the example configuration. They do not control how quickly your server opens outbound connections to Gmail or another destination, so changing anvil limits will not cure outbound 421 deferrals.
Concrete outbound concurrency values depend on the receiving domain and your traffic pattern. Reduce concurrency and delivery rate for the affected destination, observe the response, and change one control at a time. Keep a written record of the old and new settings so you can reverse an overcorrection.
Allow a legitimate scanner narrowly
TLS scanners can create bursts that trip connection limits. If a known scanner is being blocked, use a narrow smtpd_client_event_limit_exceptions entry for that scanner only, with verified source addresses. Never create a broad exception for an entire network range without confirming ownership and necessity.
A practical repair sequence
- Capture a baseline: send a new test to Gmail, save Authentication-Results, note the queue ID, and classify the outcome as spam, rejection, or deferral.
- Correct SPF: leave exactly one
v=spf1TXT record, authorize real senders, and bring DNS lookups under 10. - Correct DKIM: match the selector in the signature to the public-key DNS name, verify the private key, and confirm a 2048-bit key where practical.
- Deploy DMARC monitoring: use
p=none, read reports, and repair alignment before enforcement. - Verify the path: decide whether Google Workspace or your own Postfix is the actual relay, then authorize and sign that path only.
- Fix identity: set PTR, forward DNS,
myhostname, and a trusted STARTTLS certificate. - Inspect reputation: review complaints, suppression lists, Postmaster data, and blocklists.
- Separate throttling: analyze 421 responses and queue growth by destination, then slow only the affected outbound stream.
- Retest: send newly generated messages to Gmail and at least one other major mailbox provider; compare headers and logs after DNS caches have refreshed.
- Escalate with evidence: when a rejection remains, provide the recipient’s exact SMTP code, timestamp, sending IP, queue ID, and authentication results to your hosting provider or the recipient’s postmaster channel.
Common failure patterns and their fixes
| Observed result | Likely cause | Fix |
|---|---|---|
spf=permerror |
Multiple SPF records or more than 10 DNS lookups. | Merge records and remove or redesign unnecessary includes. |
dkim=fail after a key change |
Selector, domain, or public-key DNS mismatch. | Compare the s= and d= values in the header with the exact DNS name. |
SPF and DKIM pass but dmarc=fail |
Neither authenticated domain aligns with visible From. | Align the DKIM signing domain or the SPF envelope domain with From. |
| Authentication passes, but Gmail still files Spam | Complaint, content, list-quality, IP, or domain-reputation problem. | Reduce unwanted mail, honor suppression requests, inspect reputation data, and keep streams separated. |
| Repeated 421 “unusual rate” | Outbound concurrency or volume is too high for that destination. | Slow that destination’s delivery and monitor retries; do not change inbound anvil limits. |
| Immediate rejection despite valid authentication | Missing PTR, blocklist listing, policy violation, or poor IP reputation. | Verify reverse DNS and blocklists, then use the complete rejection text to identify the policy. |
What to monitor after the fix
- Authentication-Results for representative messages from every sending application.
- DMARC aggregate reports, especially new source IPs and alignment failures.
- Postfix queue depth, deferred-message age, and destination-specific 4xx rates.
- Hard bounces, user complaints, unsubscribe latency, and suppression-list accuracy.
- PTR and certificate expiry after every provider, IP, or hostname change.
- Blocklist status for every active sending address, including IPv6.
The durable solution is a verified identity and a controlled sending pattern: SPF, DKIM, and DMARC aligned to From; PTR and forward DNS that identify the server; trusted TLS; low complaint rates; and outbound delivery slow enough for each recipient domain. Once those controls are correct, Postfix becomes the transport layer it is meant to be rather than the place where a reputation problem is hidden.
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