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Postfix is rarely the sole reason a message lands in spam. Gmail and other mailbox providers judge the complete delivery identity: SPF and DKIM authentication, DMARC alignment, reverse DNS, TLS, IP and domain reputation, complaint rates, message volume and delivery behavior. Fix those signals in that order, then use the message headers and Postfix logs to distinguish spam classification from rate throttling or a hard rejection.
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First, identify what “spam” means in your case
People often describe three different outcomes as “Postfix mail going to spam.” The remedy depends on which one you are seeing.
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| Observed result | What it usually indicates | Where to investigate |
|---|---|---|
| The message arrives in Gmail Spam | Gmail accepted the SMTP transaction but classified the message as unwanted. | Authentication-Results, From-domain alignment, content, complaints and reputation. |
| A 421 response or a Postfix “deferred” queue entry | The receiving service is temporarily slowing or rate-limiting your server. | Postfix logs, delivery concurrency, retry behavior and the receiving domain’s response text. |
| A 550/5xx rejection | The receiving service permanently refused the message for an authentication, policy, reputation or recipient reason. | The complete SMTP reply, DNS records, blocklist status and sender policy. |
Do not try to solve a 421 “unusual rate” response by changing the message’s HTML, and do not try to solve a spam-folder classification by raising Postfix concurrency. They are separate problems.
The correct order for fixing Postfix deliverability
- Send a controlled test to a Gmail mailbox you can inspect.
- Read the message’s
Authentication-Resultsheader and record SPF, DKIM and DMARC results. - Correct SPF so there is one valid policy and no more than 10 DNS lookups during evaluation.
- Correct DKIM signing and make the selector in Postfix/OpenDKIM match the DNS selector exactly.
- Publish DMARC, begin with monitoring, and tighten the policy only after alignment is stable.
- Ask the hosting provider to set a PTR record that maps the sending IP to the server’s real hostname.
- Use STARTTLS with a certificate from a trusted certificate authority.
- Check IP and domain reputation, user complaints and common DNS blocklists.
- Separate outbound throttling from spam classification by reading the queue and delivery logs.
Most of this work happens in DNS, your VPS or hosting control panel, your signing service and your sending practices. Editing main.cf alone cannot create a valid SPF record, publish a DKIM key, set reverse DNS or repair a damaged reputation.
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Inspect a real Gmail message before changing anything
Find the authentication results
Send a message with the same visible From: domain and the same Postfix path used in production. In Gmail, open the message, choose “Show original,” and locate Authentication-Results. Record each result as pass, fail, neutral, softfail or none, along with the domains and selector shown.
A useful result identifies the authenticating domain and the alignment decision, not just a generic “mail accepted” line. A message can have SPF pass for its envelope sender while DMARC fails because that envelope domain does not align with the visible From domain. Likewise, DKIM can pass for a different signing domain and still fail DMARC alignment.
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For a rejection or deferral, copy the complete SMTP response from the Postfix log. The status code, enhanced status code and explanatory text are more useful than a screenshot of a mail client. A response mentioning authentication, reverse DNS or policy points to a different fix than one mentioning an unusual rate or temporary resource limit.
SPF: authorize every legitimate sending path
Publish exactly one SPF policy
SPF is a TXT record on the envelope-from domain. It lists the IP addresses and services allowed to send mail for that domain. Publish one record beginning with v=spf1. Two SPF TXT records at the same name cause a permanent SPF error rather than combining safely.
A simple self-hosted example might look like this:
example.com. 3600 IN TXT "v=spf1 ip4:203.0.113.25 -all"Replace the example address with your actual sending IP and include every other legitimate sender before using a hard fail. If a third-party service also sends for the domain, its documented mechanism must be added or the service should use a separate sending subdomain.
Stay below the DNS-lookup limit
The SPF evaluation limit is 10 DNS lookups, as specified by RFC 7208 and restated by a 2026 hosting guide. Mechanisms such as include, a, mx and redirect can consume lookups, including lookups nested inside an include. Exceeding the limit produces a permanent error even when every individual provider is legitimate.
When many systems send mail, use separate subdomains where practical—for example, one subdomain for an application and another for marketing—so each policy remains small and understandable. Do not add random includes copied from old providers that no longer send your mail.
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Verify the record from the public DNS
dig +short TXT example.comRun the query against public resolvers as well as the resolver used by your server. Remove old or duplicate SPF records, allow DNS propagation, then send another test. SPF authenticates the envelope sender; DMARC still requires alignment with the visible From domain.
DKIM: make the signature and DNS selector agree
Check the selector name
DKIM adds a cryptographic signature to the message. The receiving server uses the selector in the signature to find the public key in DNS. If the signature says s=selector1, the public key must be published at:
selector1._domainkey.example.comA selector mismatch is a common silent failure: Postfix or OpenDKIM may sign successfully, but the receiver cannot find the corresponding key. Compare the selector in the generated DKIM-Signature header with the selector in your signing configuration and DNS record character for character.
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A 2048-bit key is the common choice for a new deployment. Publish the complete public key as a TXT record, preserving the key value without accidental line breaks or quotation errors introduced by your DNS provider. Keep the private key readable only by the signing service account and rotate it by introducing a new selector, verifying it, then retiring the old selector.
Inspect a test message for dkim=pass. A pass for a signing domain that does not align with the visible From domain can still leave DMARC failing, so inspect the d= value as well.
DMARC: align the visible From domain with SPF or DKIM
Understand what DMARC evaluates
DMARC checks whether SPF or DKIM both authenticates and aligns with the domain shown to the reader in the From header. A message can therefore have SPF pass and DKIM pass yet fail DMARC if neither authenticated domain aligns with that visible domain.
Start with monitoring
Begin with a TXT record such as:
_dmarc.example.com. 3600 IN TXT "v=DMARC1; p=none; rua=mailto:[email protected]"p=none requests reporting without telling receivers to quarantine or reject messages. Use aggregate reports to find forgotten applications, forwarding paths and unauthorized senders. Confirm that every legitimate stream passes alignment before enforcing a stricter policy.
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Move gradually to enforcement
After monitoring shows stable alignment, move toward p=quarantine and eventually p=reject, changing one stage at a time. A policy that is too strict before all senders are aligned can block password resets, invoices or application alerts. DMARC policy affects receivers; it does not repair a message that already has poor content or a bad sending reputation.
Google Workspace relay versus self-hosted Postfix
A Postfix server relaying through Google Workspace is configured differently from a server delivering directly from its own IP. A Workspace relay commonly requires the Google SPF include include:_spf.google.com, DKIM signing enabled in the Workspace Admin console and a DMARC record for the From domain. Google controls the signing path and relay authorization.
With self-hosted Postfix, you must sign with your own DKIM key, authorize your own public IP addresses in SPF and ensure that the envelope and From domains align. Do not copy a Workspace SPF include into a direct-delivery server unless Google is genuinely an authorized sender. Conversely, do not assume that enabling Workspace DKIM signs mail that bypasses Workspace and leaves directly from your VPS.
PTR and forward DNS: give the sending IP a consistent identity
The public sending IP should have a PTR (reverse-DNS) record that resolves to the hostname used by the mail server. The forward record for that hostname should resolve back to the same IP. Large providers commonly penalize or reject mail from addresses without coherent reverse DNS.
PTR is normally controlled by the hosting or transit provider, not by the authoritative DNS zone where you manage your domain. Ask the provider to set the reverse record, for example:
203.0.113.25 -> mail.example.comThen verify both directions:
dig +short -x 203.0.113.25
dig +short A mail.example.com
hostname -fUse one stable hostname in Postfix’s myhostname, in the SMTP greeting and in the TLS certificate name. Changing the hostname repeatedly while keeping the same IP creates an inconsistent identity and makes troubleshooting harder.
TLS: encrypt the SMTP connection with a trusted certificate
Configure Postfix to offer STARTTLS and use a certificate issued by a publicly trusted certificate authority. Self-signed certificates may encrypt traffic in some controlled relationships, but they do not provide the trust expected for general Internet delivery and can contribute to policy failures.
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Check that the certificate covers the hostname presented by your server, that the chain is complete and that it has not expired. A typical inspection command is:
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Reputation, complaints and unsubscribe handling
Keep complaint rates low
Mailbox providers learn from user actions. A technically authenticated message can still be placed in spam when recipients frequently report it, delete it unread or rarely engage. Send only to people who expect the mail, remove invalid addresses promptly and stop sending to recipients who repeatedly bounce or never engage.
Google Postmaster Tools guidance has been restated by Email Awesome in 2026 as a 0.3% user-reported spam-rate ceiling. Treat that figure as an operational warning, not a timeless guarantee: Google’s own current sender and Postmaster documentation should be checked because enforcement criteria change.
Make marketing mail easy to leave
Promotional mail should include a visible unsubscribe method and support one-click unsubscribe headers where applicable. The cited guidance expects unsubscribe requests to be processed promptly, within two days. Do not force a recipient to log in, answer a survey or send a separate email just to stop marketing messages.
Build reputation deliberately
- Use a stable From domain and a consistent sending IP where possible.
- Separate transactional mail from bulk marketing so a complaint spike in one stream does not damage every message.
- Increase volume gradually after a long idle period or an IP change.
- Monitor bounces, complaints and deferrals by recipient domain.
- Remove compromised accounts or scripts immediately; unexpected bursts can damage reputation faster than normal campaigns.
Published material has used conflicting descriptions of the volume at which Google treats a sender as a bulk sender. Do not rely on an old “5,000 messages per day” figure without checking Google’s current documentation. Google’s help guidance distinguishes requirements for all senders—SPF or DKIM—from bulk senders, for whom SPF, DKIM and DMARC are expected.
Independent reader supportYour contribution helps us test, update, and keep practical guides available for everyone.Check DNS blocklists, but do not mistake a listing for the whole diagnosis
Query the sending IP and, where relevant, the sending domain against widely used DNS-based blocklists. A listing can explain a 5xx rejection or poor placement, but an unlisted IP does not prove that authentication, alignment or reputation are healthy. The exact lookup procedure and delisting rules vary by operator.
If listed, identify the cause before requesting removal: an exposed relay, compromised mailbox, weak SMTP authentication, malware or a sudden unexplained volume increase. Stop the abuse, secure the account or host, preserve relevant logs and follow that blocklist’s documented removal process. Repeatedly requesting delisting without fixing the cause usually results in another listing.
Separate outbound throttling from spam classification
Read the Postfix queue and logs
Postfix’s anvil limits, such as an example smtpd_client_connection_rate_limit = 30, govern inbound client connections to your server. They do not control how quickly your server delivers outbound mail to Gmail. Raising or lowering an anvil setting will not fix outbound 421 deferrals.
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postqueue -p
grep 'status=deferred' /var/log/mail.log | tail -n 50
grep -Ei '421|unusual rate|thrott|status=bounced' /var/log/mail.log | tail -n 100Use your distribution’s actual mail-log path if it differs. A 421 “unusual rate” response means the recipient is asking you to slow down temporarily. Review the destination-specific response, reduce outbound concurrency or delivery rate for that domain, and allow Postfix’s normal retry schedule to work. There is no single safe concurrency number verified for every provider; tune from the provider response and your logs rather than copying a universal value.
Do not disable retries blindly
Temporary deferrals should remain in the queue for retry. Deleting the queue can turn a temporary capacity problem into permanent message loss. A permanent 550 response requires a different action—usually correcting authentication, policy, recipient validity or reputation—before retrying.
TLS scanners and connection-rate side effects
Security scanners, including TLS testing services, can open many connections in a short period and trigger inbound rate limits. If a scanner is legitimate and its addresses are known, a narrow exception such as smtpd_client_event_limit_exceptions can be considered for that scanner only. Do not create a broad exception or disable connection limits for the Internet; that can expose the server to abuse and undermine the protection the limits provide.
A repeatable verification workflow
- Establish the baseline. Send one message to Gmail and one to another major mailbox provider. Save the complete headers and the Postfix queue ID.
- Validate SPF. Confirm one TXT record, all legitimate senders, correct IP mechanisms and fewer than 10 DNS lookups.
- Validate DKIM. Match the selector in the header to the DNS name, confirm the public key is complete and verify a 2048-bit key where supported.
- Validate DMARC. Confirm the visible From domain aligns with SPF or DKIM, collect reports with
p=none, then enforce gradually. - Validate identity. Check PTR, forward DNS, Postfix hostname, SMTP greeting and certificate name.
- Validate transport. Confirm STARTTLS works and the certificate chain is trusted and current.
- Validate reputation. Review complaint data, bounces, Postmaster data where available and common blocklists.
- Classify the failure. Use the exact 421, 450, 550 or 5xx response and queue state to decide whether to throttle, correct policy or secure the host.
- Retest after propagation. DNS changes and reputation corrections are not instantaneous. Recheck headers after public DNS has updated and compare results over multiple messages rather than one isolated sample.
Common mistakes that keep mail in spam
- Publishing two SPF records because different administrators added one independently.
- Adding so many SPF includes that the 10-lookup limit is exceeded.
- Rotating a DKIM selector in Postfix but forgetting to publish the new selector’s key.
- Assuming SPF pass automatically means DMARC pass.
- Using a visible From domain that differs from the authenticated envelope or DKIM domain.
- Expecting a self-signed certificate to establish Internet-wide trust.
- Trying to set PTR in the ordinary DNS control panel instead of asking the IP provider.
- Increasing Postfix outbound concurrency in response to a 421 rate response.
- Sending marketing mail without a fast, functional unsubscribe path.
- Changing several variables at once, leaving no way to tell which correction worked.
When to escalate to the hosting provider or mailbox provider
Ask the hosting provider to intervene when you cannot set or correct PTR, when port 25 is filtered, when the assigned IP is already listed or when abuse originating from the host is suspected. Provide the IP, hostname, queue IDs, timestamps and exact remote responses.
Use the receiving provider’s postmaster or sender-support channel when authentication passes, reverse DNS is coherent, the server is not listed and the problem persists across a sustained period of legitimate low-volume sending. Include headers and SMTP responses, not just a claim that messages are “in spam.”
Bottom line
Fix Postfix deliverability from the outside in: authenticate the From domain with SPF, DKIM and aligned DMARC; give the IP a matching PTR and trusted STARTTLS identity; control complaints and sending volume; check blocklists; then tune outbound delivery only when logs show throttling. Once every test message shows aligned authentication and the logs no longer show policy or rate errors, spam placement becomes a reputation and recipient-engagement problem rather than a missing main.cf switch.
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