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Postfix is rarely the sole reason a message lands in spam. Gmail and other receiving systems score the sending domain, IP address, authentication, message behavior and complaint history. Fix delivery in this order: verify the headers, publish correct SPF, sign with DKIM, align DMARC, set matching reverse DNS, use trusted TLS, check blocklists and reputation, then investigate throttling separately. Changes to main.cf cannot compensate for a missing DNS record or a poor sending reputation.
First determine what “spam” means
Ask the recipient for the complete delivery result, not just a screenshot of a folder. There are three different failure modes:
- Spam classification: the message is accepted with a
250-class response but placed in Junk or Spam. Authentication, reputation, complaints and message signals are the usual causes. - Rejection: the receiving server returns a permanent error, commonly a
550-class response. The message is not queued for later delivery. - Temporary deferral or throttling: the receiver returns a
421response such as “unusual rate.” Postfix keeps retrying according to its queue policy. This is a rate or concurrency problem, not the same as spam placement.
Keep the SMTP transcript or Postfix log entry for each case. A bounce reason, enhanced status code and receiving host name are more useful than the destination folder alone.
Use a Gmail test to establish the baseline
Send a message from the real production address to a Gmail mailbox that you control. In Gmail, open the message, choose Show original, and inspect the Authentication-Results header. You want to see successful results for SPF, DKIM and DMARC, with the domains aligned to the address shown in the visible From: header.
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Authentication-Results: ...
spf=pass ...
dkim=pass header.d=example.com
dmarc=pass header.from=example.comRecord the envelope sender (the SMTP MAIL FROM or Return-Path), the DKIM signing domain (d=), the selector, the connecting IP and the visible From domain. A message can show “SPF pass” and still fail DMARC if SPF authenticated a different domain. The same is true when DKIM passes for a domain unrelated to the From address.
Run this check after every DNS or signing change. DNS caches and queued mail can make a change appear inconsistent for a while, so test a newly generated message rather than relying on an old copy.
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Sender Policy Framework authorizes the IP addresses and services allowed to send for a domain. Publish exactly one TXT record whose value begins with v=spf1. Two SPF records for the same domain produce a permanent SPF error; merge the mechanisms into one record instead of adding a second.
An SPF evaluation is limited to 10 DNS-lookup-causing mechanisms under the IETF SPF standard (RFC 7208). Nested include, redirect, a, mx and similar mechanisms all count. Large records assembled from many marketing, help-desk and cloud services can exceed that limit even when the text itself is short.
Check the record and the actual sending IP
- Identify the public IPv4 and IPv6 addresses that connect to the recipient. NAT, a relay or a second Postfix host may mean the address is not the one you expected.
- Query the DNS record, for example
dig +short TXT example.com. - Confirm that one returned TXT value starts with
v=spf1and authorizes the connecting address, either directly (such asip4:orip6:) or through a deliberately chosen include. - Send another test and verify that SPF passes for the envelope-from domain shown in the results.
If several independent services send mail, a dedicated sending subdomain for each service can keep each SPF policy below the lookup limit. Do not add an include merely because a vendor offers one; every include expands the evaluation and must be maintained when that service changes its infrastructure.
SPF does not replace DKIM or DMARC
SPF authenticates the envelope sender, which recipients do not normally see. Forwarding can also break SPF. Treat a passing SPF result as one input to DMARC, not as proof that the visible From address is trusted.
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Make DKIM signing and DNS agree
DomainKeys Identified Mail adds a cryptographic signature to the message. Postfix commonly hands signing to a milter such as OpenDKIM, although the exact architecture depends on your distribution. The public key is published at a DNS name formed from the selector and ._domainkey; a selector named selector1 therefore uses selector1._domainkey.example.com.
Verify the selector, key and signature
- Read the selector configured in the signing service and in the Postfix milter configuration.
- Query the matching public key, for example
dig +short TXT selector1._domainkey.example.com. - Check that the DNS key is complete, has not been split incorrectly by the provider, and corresponds to the private key used by the signer.
- Send a new message and inspect
DKIM-SignatureandAuthentication-Results. The result should showdkim=passand the expectedheader.d=domain.
A selector/DNS mismatch commonly fails without an obvious Postfix error: the message leaves the server, but the recipient cannot retrieve the advertised key. A 2048-bit key is the common choice for a new deployment. Rotate keys by publishing a new selector, switching the signer, waiting for caches to expire, and removing the old selector only after mail signed with it can no longer be in transit.
Sign the headers that identify the message, including the From header. If a downstream system modifies a signed header or the body, DKIM can fail; inspect which hop made the change before disabling signing.
Deploy DMARC in monitoring mode, then enforce alignment
DMARC checks whether SPF or DKIM both authenticates and aligns with the visible From domain. Alignment is why a message can have SPF pass but DMARC fail: the envelope-from domain may be different. DKIM can provide the aligned path when forwarding breaks SPF.
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A safe policy progression
- Publish a DMARC TXT record at
_dmarc.example.comwithp=noneand an aggregate-report destination you control, such asrua=mailto:dmarc-reports@example.com. - Review aggregate reports for every legitimate sender. Identify services using the domain, their source IPs and whether SPF or DKIM is aligned.
- Correct missing authorization or signing, then continue monitoring long enough to cover normal and seasonal traffic.
- Move selected traffic toward
p=quarantine, and ultimatelyp=reject, only when legitimate sources consistently pass alignment. Use percentage or subdomain controls where your policy design requires a gradual rollout.
p=none is monitoring-only; it does not ask a receiver to quarantine or reject a message. Protect the report mailbox because aggregate reports can contain addresses, sending sources and volume information.
Google’s sender guidance distinguishes requirements for all senders from those for bulk senders: all senders are expected to use SPF or DKIM, while bulk senders are expected to use SPF, DKIM and DMARC. The definition and enforcement details for bulk traffic change, so check Google’s current sender documentation before relying on any particular daily-volume threshold.
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Set reverse DNS (PTR) that matches the hostname
The connecting IP should have a PTR record that resolves to the hostname used by the mail server, and that hostname should resolve back to the same IP (forward-confirmed reverse DNS). For example, a connection from 203.0.113.10 should reverse to mail.example.com, and mail.example.com should contain 203.0.113.10.
dig +short -x 203.0.113.10
dig +short A mail.example.com
dig +short AAAA mail.example.comThe IP owner, usually your hosting or transit provider, controls the reverse-DNS zone. You normally cannot create the PTR record in your ordinary domain DNS panel; open a provider request or use the provider’s reverse-DNS control. Large mailbox providers often penalize or reject mail from addresses without usable reverse DNS. Avoid changing the hostname in Postfix until the provider has set the PTR and forward record consistently.
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Receiving systems expect mail to be delivered over encrypted SMTP when the path supports it. Enable STARTTLS and use a certificate issued by a publicly trusted certificate authority for the server name. A self-signed certificate may encrypt a connection but does not provide the identity validation receiving systems expect.
For an ordinary Postfix installation, opportunistic outbound TLS is commonly enabled with a setting such as smtp_tls_security_level = may; inbound policy and certificate paths use different smtpd_ settings. Parameter names and safe defaults vary by Postfix version and operating system, so verify the syntax with postconf and your distribution’s documentation. Do not force a mode that prevents delivery to destinations that legitimately do not offer STARTTLS, and do not treat TLS success as a substitute for authentication.
Protect reputation and reduce complaints
Authentication proves who sent a message; reputation determines whether the receiver wants to accept it. Keep complaint rates low by sending only to people who requested the mail, removing invalid or persistently inactive addresses, and separating transactional traffic from marketing traffic. Sudden volume spikes can trigger temporary rate controls even when SPF, DKIM and DMARC are correct.
Google Postmaster Tools guidance is often summarized as a 0.3% user-reported spam-rate ceiling. That figure was restated by Email Awesome in 2026; Google’s current documentation should be checked before treating it as a contractual limit, because enforcement and measurement can change. Watch the trend, not just a single day’s value, and investigate a rise by campaign, list source and sending IP.
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Marketing messages should include a one-click unsubscribe mechanism and process the request promptly; guidance cited in the available material describes processing within two days. Do not require a login or make the unsubscribe link harder to use than the subscription. Transactional notices can still be reported as spam when recipients did not expect them, so make the purpose and sender identity clear.
Check blocklists before rebuilding the server
Query the sending IPv4 and IPv6 addresses against the common DNS-based blocklists used by mailbox providers. A listing does not prove that every provider will reject you, but it is an important lead. Record the list name, listing reason, timestamp and delisting procedure. Fix the underlying cause—compromised credentials, an open relay, malware, a misdirected application or abusive list—before requesting removal.
Blocklist status and spam classification are separate. A clean result does not guarantee inbox placement, and a listing may explain a rejection even when authentication passes.
Independent reader supportYour contribution helps us test, update, and keep practical guides available for everyone.Separate outbound throttling from spam placement
Postfix logs reveal whether the receiver accepted, rejected or deferred a message. Search for the destination response and the queue ID:
grep -E "status=|421|450|451|550|554" /var/log/maillog
postqueue -pPaths differ by distribution; some systems use /var/log/mail.log instead of /var/log/maillog. A response such as 421 unusual rate means the destination is temporarily limiting your delivery. Slow outbound delivery to that domain, reduce parallel connections using a per-destination transport policy, and watch retries until the response clears. There is no universally safe concurrency number: the right setting depends on destination policy, your traffic pattern and your Postfix version.
The Postfix anvil parameter smtpd_client_connection_rate_limit limits inbound client connection rates to your server. An example value such as 30 concerns clients connecting to you; it does not control how quickly your server connects to Gmail or another remote destination. Changing it will not cure outbound 421 deferrals.
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When a queue grows, inspect whether one destination is responsible, whether retries are aging out, and whether a DNS or network failure affects multiple domains. Do not repeatedly flush the queue while a receiver is rate-limiting you; that can create another burst and prolong the deferral.
Choose the correct setup for your mail path
Self-hosted Postfix
Your own server must sign with your own DKIM key, authorize its public IP addresses in SPF, publish DMARC for the visible From domain, obtain matching PTR, and present a valid TLS certificate. A relay or application that rewrites the envelope sender can change which SPF record is evaluated, so test the final Internet-facing hop.
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A Workspace relay arrangement usually involves an SPF include for Google’s sending infrastructure (include:_spf.google.com), DKIM signing enabled in the Admin console, and a DMARC TXT record for the From domain. Confirm the actual connecting IP and signing domain in the received headers; do not copy Workspace settings into a self-hosted Postfix path unless Workspace is truly the system sending the message.
Use a repeatable diagnosis checklist
- Capture evidence: save the full Gmail “Show original” output and the matching Postfix log entry.
- Check blocklists: test every public sending address, including IPv6.
- Check SPF: one
v=spf1record, correct IP authorization and fewer than 10 DNS lookups. - Check DKIM: selector, public key, private key and aligned signing domain all match.
- Check DMARC: confirm that SPF or DKIM aligns with the visible From domain; review aggregate reports.
- Check PTR and TLS: verify forward-confirmed reverse DNS and a trusted certificate with STARTTLS.
- Check reputation: review complaint trends, list hygiene, unsubscribe processing and sending spikes.
- Classify the response: treat a 550 rejection, spam placement and a 421 deferral as different incidents.
- Retest: send a newly generated message after each material change and compare headers, response codes and placement.
Handle TLS scanner rate limits without opening a broad exception
Security scanners can make many rapid SMTP connections and trigger client-event limits. If a legitimate scanner is being limited, use a narrow allowlist such as Postfix’s smtpd_client_event_limit_exceptions for the scanner’s documented addresses only. Keep the exception specific, review the addresses periodically, and never disable rate controls for the entire Internet.
Why common “fixes” fail
- Editing only
main.cf: DNS authorization, DKIM keys, PTR and reputation live outside that file. - Adding a second SPF record: receivers treat multiple SPF records as a permanent error.
- Changing the DKIM selector without publishing its key: verification fails even though Postfix continues to send.
- Setting DMARC to reject immediately: legitimate senders that have not been inventoried can be rejected.
- Increasing outbound concurrency after a 421: a rate-limited destination usually needs slower, steadier delivery.
- Assuming TLS proves legitimacy: encryption protects the path; it does not authenticate the sender domain.
- Testing only one message: forwarding, alternate IPs, IPv6 and different envelope senders can produce different results.
When authentication passes, reverse DNS is correct, the IP is not listed, complaint rates are controlled and the logs show successful acceptance rather than deferral, remaining spam placement is primarily a receiver reputation and content decision. At that point, compare campaigns and recipient engagement instead of making random Postfix configuration changes.
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