Most DKIM failures in Webmin or Virtualmin come from one of four mismatches: OpenDKIM is absent or misconfigured, signing is disabled, the DNS record uses the wrong selector, or the TXT record was added to a DNS server that is not authoritative for the domain. Fix the problem by tracing the complete path: confirm which server sends mail, identify the authoritative nameservers, repair OpenDKIM, enable signing, publish the exact public key at selector._domainkey.example.com, and test that record from the public DNS system.
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What a DKIM failure means
DomainKeys Identified Mail (DKIM) adds a cryptographic signature to outgoing email. The mail server keeps a private key and places a signature in the message headers. Receiving servers retrieve the matching public key from DNS and use it to verify that the message was signed by the stated domain and was not altered in transit.
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The DNS name is built from two values in the DKIM-Signature header: the signing domain (d=) and selector (s=). For a message containing d=example.com and s=2026-10, the verifier looks up:
2026-10._domainkey.example.com
A valid private key with a missing public record, a typo in either value, a stale key after rotation, or a record published on the wrong DNS service all produce verification errors. Virtualmin can create and use DKIM only for a virtual server that has both its DNS and email features enabled.
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1. Map the DNS and mail architecture first
Do not begin by editing a zone file. First establish where each part of the system lives. Virtualmin may host the website and mail while another provider hosts DNS. In that arrangement, a record added to the local BIND zone is not visible to the Internet because public resolvers follow the nameservers delegated at the registrar.
Questions to answer
- Does this Virtualmin server receive and send mail for the domain?
- Is DNS hosted by the same server, by another server you operate, or by an external DNS provider?
- Which nameservers are delegated for the domain?
- Does the Virtualmin virtual server have both DNS and email features enabled?
- Which selector appears in the current outgoing message’s
DKIM-Signatureheader?
Use the right authority
If the domain’s delegated nameservers point to the Virtualmin server, publish the record in the BIND zone managed through Webmin’s BIND DNS Server module or Virtualmin’s DNS tools. If delegation points elsewhere, create the record at that DNS provider instead. The local zone editor can show a perfectly formed entry while public resolvers continue receiving an entirely different zone.
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2. Install or repair OpenDKIM
Virtualmin’s DKIM page depends on OpenDKIM. If the interface reports a missing configuration file, an unavailable module, or that DKIM cannot be configured, repair the package installation before changing DNS.
Install from the Virtualmin interface
- Open Email Messages and choose DomainKeys Identified Mail. Depending on the Virtualmin release, the same controls may appear under Email Settings.
- If Virtualmin reports that OpenDKIM is missing, select Install Now.
- After installation, return to the DKIM page and confirm that the configuration paths and signing controls are available.
Install from the operating system
On Enterprise Linux systems, the documented package command is:
dnf install opendkim
On Debian-derived systems, use:
apt-get install opendkim
These commands install the package; Virtualmin still has to know where the configuration files and keys are located. Nonstandard packages or customized layouts may require you to enter those paths in the Webmin module settings.
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If installation fails
- Read
/root/virtualmin-install.logfor the package and module error that stopped the installation. - Confirm that the configured repositories are reachable and contain metadata for your operating-system release.
- Check network access, DNS resolution, and repository authentication from the server.
- Look for third-party or locally rebuilt packages that changed the expected OpenDKIM file locations.
- After correcting the package source, run the installation again and reopen the Virtualmin DKIM page.
Do not publish a guessed public key while OpenDKIM is incomplete. The key shown by Virtualmin must correspond to the private key that the active mail path actually uses.
3. Enable signing and record the active selector
Once OpenDKIM is available, enable signing for the specific virtual server. A domain can have a working mail transport and still send unsigned messages when this setting is disabled.
Webmin or Virtualmin interface
- Open Email Settings → DomainKeys Identified Mail (or the equivalent Email Messages → DomainKeys Identified Mail page).
- Set Signing of outgoing mail enabled? to Yes.
- Enter a selector, such as a year-and-month value. Keep a note of the exact spelling, capitalization, and punctuation.
- Save the configuration.
- Copy the DNS publication instructions and the complete public-key value displayed by Virtualmin.
Virtualmin generates or uses the private key associated with that selector. The selector in this page must match the s= value later seen in a test message; a record for a different selector cannot validate the signature.
Command-line controls
The documented CLI control to enable signing is:
virtualmin set-dkim --enable
Use --select name to choose a selector and --size bits to set the key size. Incoming-signature checking is controlled separately with --verify or --no-verify. The command-line and GUI settings should describe the same virtual server; changing one and inspecting another is a common source of confusion.
By default, incoming email is not checked for a valid DKIM signature unless --verify is supplied. Signing outgoing mail and verifying incoming mail are separate decisions.
4. Publish the exact public key in DNS
Create a TXT record at the selector name Virtualmin actually configured. If the selector is 2026-10 and the domain is example.com, the record name is:
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2026-10._domainkey.example.com
Copy the value without editing it
Use the complete value supplied by Virtualmin, including the v=DKIM1 and p= fields. Do not add quotation marks that your DNS provider does not require, remove punctuation, insert line breaks into the key, or substitute a key from an earlier rotation. A single missing character makes the public key fail to match the private key.
Publishing in local BIND
When the Virtualmin server is authoritative, add the TXT record through Webmin’s BIND DNS Server module or the Virtualmin DNS editor. Set the record name, type, TTL, and value, then apply the zone change. The Virtualmin modify-dns API exposes those same record fields—name, type, TTL, and value—if you manage zones programmatically.
Publishing at an external DNS provider
When the domain’s NS records point to an external provider, make the change in that provider’s zone editor. Editing the local BIND zone will not change the answer received by outside mail systems. Managed DNS can simplify nameserver, TTL, and propagation administration, but it does not remove the need to verify delegation.
Long TXT values and split strings
DNS interfaces may display a long DKIM key as several quoted TXT strings. That is normal when the provider enforces a per-string limit. Preserve every segment in order. A resolver concatenates the strings for DKIM use; your test with dig should contain the complete key when the segments are joined.
5. Test the public DNS path
Use public DNS queries rather than relying on the zone file or a local resolver cache.
Basic lookups
Query the TXT record with:
dig TXT selector._domainkey.example.com
For a simpler response, use:
host -t TXT selector._domainkey.example.com
Replace the selector and domain with the exact values from Virtualmin. An empty answer, an old key, or a response from an unexpected provider points to a DNS publication or delegation problem rather than an OpenDKIM signing problem.
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Ask the authoritative nameserver directly
If different recursive resolvers return different answers, identify the delegated nameservers and query one directly:
dig @ns1.example.net TXT selector._domainkey.example.com
This separates cached data from the answer currently served by the authority. Query each delegated nameserver when necessary; inconsistent answers indicate that the zone is not synchronized.
Check delegation and glue
Use:
whois example.com
Confirm that the registrar lists the nameservers you expect. If a nameserver is inside the domain it serves, verify that its glue address is correct. A wrong NS or glue record can make the entire zone—including a correctly entered DKIM record—unreachable.
DNS TTL controls how long resolvers may cache an earlier answer. During a key change, wait for the previous TTL to expire before assuming that every resolver has the new value. Always keep the old record available while messages signed with the old selector are still in circulation.
Independent reader supportYour contribution helps us test, update, and keep practical guides available for everyone.6. Confirm that mail is signing with the same selector
Send a test message through the normal Virtualmin/Postfix path to a mailbox that exposes full headers. Find the DKIM-Signature header and compare:
d=must identify the domain whose DNS zone contains the record.s=must exactly match the selector in the DNS query.- The public key returned by DNS must be the key generated for that active selector.
If there is no DKIM-Signature header, signing may still be disabled, OpenDKIM may be unavailable or misconfigured, or the message may have bypassed the configured Postfix path. Verify the Virtualmin setting and package installation before changing DNS.
If the header shows an old selector after a rotation, publish the old selector’s public key until every sender has switched, then remove it. Do not overwrite an old selector with a new key; that creates a private/public mismatch for messages already signed with the old key.
Common DKIM failure patterns
| Observed result | Likely cause | Targeted correction |
|---|---|---|
| No DKIM signature in the message | Signing is disabled; OpenDKIM is not installed or configured; or mail bypassed the configured Postfix path. | Enable signing in the virtual server, repair OpenDKIM, and confirm that the test message uses the intended mail route. |
| DKIM record not found | Selector typo, missing TXT record, wrong DNS host, or broken NS/glue delegation. | Read s= from the header, query that exact name with dig or host, then check registrar delegation with whois. |
| Public key does not match | The DNS key belongs to a previous selector or was copied incorrectly. | Republish the complete public key generated for the active selector and preserve the old selector during a controlled rotation. |
| Record appears locally but not online | Local BIND is not authoritative, an external provider serves the zone, or NS/glue records are wrong. | Publish at the delegated provider and query the authoritative nameserver directly. |
| Virtualmin cannot create DKIM configuration | Package or repository failure, missing configuration paths, or unsupported nonstandard packages. | Inspect /root/virtualmin-install.log, repair repositories or paths, and rerun the OpenDKIM installation. |
| Large key is returned as multiple TXT strings | The provider split one long value to meet DNS string limits. | Keep every string segment in order and verify the concatenated result with dig. |
Choose the right administration model
| Decision | Option A | Option B | Best fit |
|---|---|---|---|
| DNS authority | Local BIND managed in Webmin or Virtualmin | External DNS provider | Use local BIND when the server is delegated and you need one-server control. Use the external editor when the registrar delegates elsewhere. |
| DKIM administration | Webmin/Virtualmin GUI | virtualmin set-dkim CLI |
The GUI is easier to inspect interactively; the CLI is auditable and suitable for repeatable administration. |
| Incoming mail handling | Signing only | Signing plus --verify |
Signing avoids changing inbound policy. Enable verification only when you understand how your filtering and bounce behavior will respond. |
| Selector strategy | One stable selector | Overlapping selectors during rotation | A stable selector is simple. Overlap old and new selectors when replacing keys without interrupting mail delivery. |
Rotate selectors and keys without breaking mail
- Create or select a new selector in Virtualmin and generate the corresponding key pair.
- Publish the new selector’s complete public key in the authoritative DNS zone.
- Verify the new record with
digand query the authoritative nameserver directly. - Send a test message and confirm that its
s=header uses the new selector. - Keep the old selector’s TXT record while any sender, queue, or delayed message may still use it.
- After the old selector is no longer present in message headers and its DNS TTL has elapsed, remove the old record.
Changing the key value under an existing selector before senders switch is unsafe: receiving systems will retrieve the new public key while queued messages still contain signatures made with the old private key.
When to enable incoming DKIM verification
Virtualmin’s default is not to check incoming DKIM signatures. The --verify option enables that check; --no-verify disables it. Verification can help an administrator evaluate inbound authenticity, but it is not the same as a complete spam policy and should not be enabled merely to cure an outgoing-signature problem.
Before enabling it, decide how failures are handled, whether trusted internal senders use the same path, and how a failed signature will affect delivery or bounces. Keep outbound signing enabled independently so that changing inbound verification does not remove the signature from your own mail.
Quick Recap
Final diagnostic checklist
- Virtualmin’s virtual server has both DNS and email features enabled.
- OpenDKIM is installed from the Virtualmin installer or the correct system package command.
- Any package or path error has been reviewed in
/root/virtualmin-install.log. - Outgoing signing is enabled and the selector is recorded exactly.
- The TXT record is published at
selector._domainkey.domainon the delegated authoritative DNS service. - The complete
v=DKIM1andp=value is intact, including split TXT segments. dig,host, and, when needed, direct authoritative queries return the expected key.whoisconfirms the intended nameservers and glue delegation.- A real message contains a
DKIM-Signaturewhosed=ands=values match the DNS record. - Old selectors remain published until key rotation and DNS-cache lifetimes are complete.
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