To sign outgoing mail from Postfix, run OpenDKIM as a Milter and connect Postfix to it. The selector, domain, private-key path, OpenDKIM socket and DNS TXT record must describe the same key. Publish the public key at selector._domainkey.yourdomain, keep the private key readable only by OpenDKIM, configure both Postfix Milter parameters, then verify alignment with opendkim-testkey and a delivered message.
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What this setup does
DKIM (DomainKeys Identified Mail) adds a cryptographic signature to an outgoing message. A receiving server retrieves your public key from DNS, hashes the signed headers and body, and checks the signature. A passing result helps the recipient verify that the message was authorized by the domain shown in the signature and that signed content was not changed in transit.
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Postfix does not create that signature by itself in this arrangement. It hands each message to OpenDKIM through the Milter protocol. OpenDKIM reads the configured private key, creates the DKIM-Signature header, and returns the message to Postfix for delivery. Postfix’s Milter documentation describes this integration as a way to add applications that verify authenticity or digitally sign mail.
This guide covers outbound signing on Debian-family Linux VPS systems. Package names, service users, file locations, chroot settings and service-manager behavior can differ on other distributions, so confirm the installed package documentation before copying paths verbatim.
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Prerequisites and the values you must keep aligned
Have all of these ready before generating a key:
- A domain whose DNS zone you can edit.
- A working Postfix installation on a Linux VPS.
- A stable server hostname and functioning outbound SMTP configuration. The commonly cited DigitalOcean procedure assumes these are already complete.
- A domain and selector name you will use consistently. This guide uses
yourdomain.exampleandmail.
Five values form one DKIM identity. Changing one without changing the others causes a lookup or verification failure.
| Component | Example | Where it is used |
|---|---|---|
| Signing domain | yourdomain.example |
OpenDKIM Domain, key generation and the DNS owner name |
| Selector | mail |
OpenDKIM Selector, the signature’s s= value and DNS |
| Private key | /etc/dkimkeys/mail.private |
Read locally by OpenDKIM; never publish it |
| Public key | The p= value from mail.txt |
Published in a DNS TXT record |
| Milter socket | inet:8891@localhost |
OpenDKIM listener and Postfix Milter settings |
1. Install OpenDKIM and its utilities
On a Debian-family system, install the signer and test tools:
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sudo apt install opendkim opendkim-toolsThe Debian packages provide the OpenDKIM daemon plus utilities such as opendkim-genkey and opendkim-testkey. Do not proceed until the package installation completes and the service account created by your distribution is known; the examples below use the usual opendkim account.
2. Generate a selector key pair
Create the directory
sudo mkdir -p /etc/dkimkeysChoose a selector that identifies this key, such as mail or a rotation label such as 2026. Selectors let you publish a replacement key while the old selector remains available for messages already in transit.
Generate the private and public files
sudo -u opendkim opendkim-genkey -D /etc/dkimkeys -d yourdomain.example -s mailThis creates a private key (normally mail.private) and a DNS-ready public-key file (normally mail.txt). Debian documents a 2048-bit default. That is a package default, not a universal requirement. If your DNS provider cannot accept the resulting TXT length, the Debian Handbook documents generating a 1024-bit key with -b 1024; use that only when the provider limitation and your security policy justify the smaller key.
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Protect the private key
sudo chown opendkim:opendkim /etc/dkimkeys/mail.*The Debian Handbook shows ownership by opendkim. Preserve restrictive modes in your distribution’s package layout so ordinary users cannot read the private file. OpenDKIM’s RequireSafeKeys behavior can reject keys with unsafe ownership or permissions. Never put the private key in DNS, a web directory, a ticket, or a repository.
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Use the exact owner name
Open /etc/dkimkeys/mail.txt and create a TXT record with this owner name:
mail._domainkey.yourdomain.exampleCopy the complete v=DKIM1; ...; p= value generated in that file. The generated text may contain formatting intended for DNS; retain the entire public-key value, including every character after p=.
Handle long TXT values correctly
Some DNS interfaces display a long value as multiple quoted strings. That is valid when the provider concatenates the strings for DNS responses. Do not omit a segment or add explanatory text. The Debian example uses a 3600-second TTL and identifies an RSA/SHA-256 record; 3600 seconds is an example, not a mandatory TTL.
DNS systems may impose 255-character limits on an individual text string. If your provider enforces that limit, use its supported TXT-string splitting interface. A 2048-bit public key commonly needs splitting at the presentation layer; never split or alter the base64 characters themselves. If the provider cannot represent the key even with supported splitting, consult its documentation and consider the documented 1024-bit generation option only after evaluating the security trade-off.
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4. Configure OpenDKIM
Single domain and single key
For one domain and one selector, the essential settings are:
Domain yourdomain.example
Selector mail
KeyFile /etc/dkimkeys/mail.private
Socket inet:8891@localhostPlace equivalent directives in the configuration file used by your distribution, commonly under /etc/opendkim.conf. The names are case-insensitive in normal OpenDKIM configuration, but keep the spelling shown by your package documentation.
The TCP socket above listens on localhost port 8891. A local UNIX socket is also valid, for example a socket path under /run/opendkim, but Postfix must be able to reach that path. If Postfix runs chrooted, the socket has to exist inside the chroot-visible filesystem or the chroot must be adjusted. A mismatch between the configured protocol (inet: versus a UNIX path), address or port prevents signing.
Multiple domains or selectors
When one static KeyFile is no longer enough, use KeyTable and SigningTable. KeyTable maps a key name to a domain, selector and private-key path; SigningTable chooses that key from the sender identity. OpenDKIM’s manual states that defining KeyTable overrides KeyFile, and that Selector is ignored when a KeyTable is defined.
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|---|---|---|
One Domain, Selector and KeyFile |
One sending domain and one active key | Few files and the simplest troubleshooting path |
KeyTable plus SigningTable |
Several domains, identities or rotating selectors | More flexible mapping; every table entry and DNS name must match |
5. Connect Postfix to the OpenDKIM Milter
Edit /etc/postfix/main.cf and add the socket settings:
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smtpd_milters = inet:localhost:8891
non_smtpd_milters = $smtpd_milters
milter_default_action = acceptsmtpd_milters applies to mail received through an SMTP session. non_smtpd_milters applies to locally generated mail and other mail injected without an SMTP server session, such as messages submitted by a local application. Setting only the first parameter is the usual reason a locally generated message has no signature.
milter_default_action = accept tells Postfix to accept mail when the Milter is temporarily unavailable. Debian recommends this behavior so a signer outage does not discard mail. It also means a message can leave unsigned during an OpenDKIM failure, so monitor the service and authentication results rather than treating this setting as a substitute for reliability.
If you use more than one Milter, preserve the existing space-separated list and add the OpenDKIM endpoint in the syntax required by your Postfix version. The socket in Postfix must exactly match the listener configured in OpenDKIM.
Restart and reload
sudo systemctl restart opendkim
sudo systemctl reload postfixRestarting OpenDKIM ensures its key and socket settings are active. Reloading Postfix applies main.cf without unnecessarily stopping the mail queue. If your distribution uses another service manager, use its equivalent operations.
6. Validate the key and send a controlled test
Run the DNS/key alignment test
sudo opendkim-testkey -d yourdomain.example -s mail -vvvUse exactly the domain and selector configured above. A successful result indicates that OpenDKIM can find the DNS TXT record and that the local key setup is aligned according to Debian’s documentation. A verbose failure usually identifies whether the problem is a lookup, selector, domain, path or permission issue.
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Inspect a delivered message
Send a message to a mailbox you control, preferably at a different provider. View the raw message source, not only the simplified mailbox view. You should see a DKIM-Signature header containing values such as d=yourdomain.example and s=mail, followed by an Authentication-Results header reporting dkim=pass.
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The signature’s d= and s= values tell you which DNS name the recipient queried. Compare them character-for-character with the owner name you published. A message can contain a signature while still failing if the recipient cannot retrieve the matching key or if an intermediary changed signed content.
Choosing sockets, canonicalization and domain scope
These are design decisions rather than mandatory changes to the basic recipe.
| Axis | Option | When to choose it | Trade-off |
|---|---|---|---|
| Transport to OpenDKIM | TCP localhost socket (inet:8891@localhost) |
Simple package defaults and clearly separated service endpoints | Requires a listening port and matching address; confirm local firewall policy |
| Transport to OpenDKIM | UNIX socket | Local-only communication and no TCP listener | Directory ownership, path visibility and Postfix chroot handling become critical |
| Canonicalization | simple |
Strict preservation of signed headers and body formatting | More vulnerable to harmless whitespace or line-ending changes by intermediaries |
| Canonicalization | relaxed |
Mail likely to pass through systems that normalize whitespace or header folding | Allows limited normalization; it does not protect against arbitrary content edits |
| Domain scope | One domain and selector | A single controlled sending identity | Each additional domain or rotation requires a deliberate configuration change |
| Domain scope | KeyTable/SigningTable |
Multiple domains, sender identities or active selectors | More mappings to audit and more DNS records to maintain |
OpenDKIM’s manual defines the canonicalization controls and table behavior. Choose a policy that matches the transformations your mail will encounter; changing canonicalization after deployment changes the signature behavior and should be followed by a new end-to-end test.
Troubleshooting by symptom
opendkim-testkey reports “key not secure” or a lookup failure
- Confirm the TXT owner name is exactly
mail._domainkey.yourdomain.example, including the selector and domain. - Query DNS using a DNS diagnostic tool and check that the complete
p=value is returned. - Check that the command’s
-dand-svalues match OpenDKIM’sDomainandSelector. - Review private-key ownership and mode. Strict
RequireSafeKeyssettings reject unsafe files.
There is no signature on locally generated mail
Check that non_smtpd_milters = $smtpd_milters is present in main.cf, then reload Postfix. Locally generated mail follows a different Postfix path from SMTP-injected mail, so a working smtpd_milters setting alone is insufficient.
OpenDKIM cannot read the private key
- Verify the path in
KeyFileexists and is readable by the account running OpenDKIM. - Check every parent directory for execute permission by that account.
- Confirm the owner and mode after package upgrades or manual copies.
- If the daemon is restricted by a chroot or other service sandbox, make the key available in the permitted path without broadening access to other users.
Postfix cannot connect to OpenDKIM
- Ensure OpenDKIM is running and listening on the same protocol, address and port named in Postfix.
- For a UNIX socket, check socket-file ownership and whether Postfix’s chroot can see the directory.
- For the TCP example, confirm the listener is bound to localhost port 8891 and that no local policy blocks it.
- Inspect the OpenDKIM and Postfix service logs immediately after sending a test message; the first connection error usually identifies the mismatched endpoint.
The signature passes initially but fails after forwarding
Mailing-list software and other intermediaries can rewrite signed headers or body content. Debian specifically warns that these changes can invalidate DKIM. Compare the original delivered message with the forwarded copy and identify which system altered it. You cannot repair a signature after an intermediary has changed signed data; choose compatible canonicalization, avoid signing headers a known intermediary rewrites when policy permits, or accept that forwarded copies may fail.
The DNS value appears truncated
Check whether the provider split the TXT value into multiple quoted strings and whether every string is present in the DNS response. Do not paste only the first visual line. If the provider has a hard per-string limit, use its documented splitting mechanism; only then consider the documented 1024-bit key alternative, after weighing the reduced key size.
Quick Recap
Security and maintenance practices
- Back up configuration and private keys securely, separately from public DNS data. A stolen private key lets an attacker sign mail for the selector until you revoke it.
- Use a new selector for planned key rotation. Publish the new TXT record, deploy the matching private key and configuration, test a message, then retire the old DNS record only after messages signed with it have aged out.
- Limit shell and file access to administrators who need it. The private-key directory should not be writable by the web server or untrusted applications.
- Monitor for unsigned mail and authentication failures. With
milter_default_action = accept, temporary OpenDKIM outages can allow delivery without a signature. - Keep DNS TTL, selector inventory and table entries documented so a future administrator can identify which key signs which domain.
Final verification checklist
- Postfix is installed, accepts outbound mail and has a stable hostname.
- OpenDKIM and
opendkim-toolsare installed. - The private key exists at the configured path, is owned by the OpenDKIM service account and has restrictive permissions.
- The public key is published at the exact
selector._domainkey.domainowner name. - The OpenDKIM socket is listening and reachable from Postfix’s execution context.
- Both
smtpd_miltersandnon_smtpd_milterspoint to that socket. opendkim-testkey -d yourdomain.example -s mail -vvvsucceeds.- A raw delivered message contains a matching
DKIM-Signatureand reportsdkim=pass.
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