A working Resend setup has two separate layers: authenticate the sending domain in DNS, then send through the Resend API from an address that uses that domain. SPF authorizes sending servers, DKIM signs each message, and DMARC checks alignment between those results and the visible From address. Configure all three deliberately, verify them in Resend and in delivered-message headers, and treat authentication as a prerequisite for delivery—not a guarantee that mail will reach the inbox.
What SPF, DKIM and DMARC each do
These standards answer different questions for a receiving mail system. SPF checks whether the server that delivered a message is authorized to send for the envelope domain. DKIM checks a cryptographic signature created by the sender and validated with a public key in DNS. DMARC tells receivers what to do when authentication fails and where to send aggregate or forensic reports.
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SPF authorizes infrastructure
An SPF policy is a DNS TXT record for a domain. It lists the services allowed to send mail for that domain. A domain or subdomain can have only one effective SPF policy. If your SaaS also sends through another legitimate service, merge that service’s authorization into the existing policy; publishing a second SPF record creates a permanent-error condition for receivers rather than authorizing both senders.
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1Scan for outdated or missing drivers - takes under a minute2Repair Windows errors before they cause bigger problems3Fix the driver behind crashes, sound loss and screen glitchesAn SPF policy on an apex domain does not automatically cover a subdomain. If your From address uses notify.example.com, inspect and configure SPF for that sending identity as required by your provider and DNS design.
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DKIM signs message content
DKIM adds a signature header to each message. The receiving system reads the selector named in that header, looks up the matching public key in DNS, and verifies that the signed content was not altered. Providers can use different selectors and may require more than one record. Use the selector and value currently shown for your Resend domain; an example from a guide is not a production key.
DMARC connects authentication to the From address
DMARC evaluates alignment. A message passes DMARC when either SPF or DKIM passes and the authenticated domain aligns with the domain visible in the message’s From header. DMARC also publishes a policy and reporting destination, allowing you to monitor failures before asking receivers to quarantine or reject them.
Choose the sending domain before touching DNS
Decide which domain or subdomain your SaaS will place in the From address. Keeping transactional mail, product notifications and subscription or marketing streams on distinct identities can make ownership and incident response clearer. Resend recommends subdomains as a way to distinguish purposes and potentially contain reputation problems, but separation is operational guidance, not a guaranteed inbox-placement improvement.
| Identity design | Operational isolation | Reputation containment | DNS and monitoring work | When it fits |
|---|---|---|---|---|
| One domain for every stream | Low | Low | Lowest | Small systems with one consistent mail type and a small sender inventory |
| Separate subdomains by purpose | Higher | Potentially higher, not guaranteed | Higher because each identity needs records and monitoring | Products that need clear separation between transactional and subscription mail |
| Separate domains | Highest | Potentially highest, with separate reputations | Highest; ownership, authentication and reporting are split | Organizations with genuinely independent products or sending teams |
Choose a From domain that your team controls and can keep aligned across all sending paths. Record every system that may send for it: Resend, support platforms, billing tools, internal scripts and any legacy provider.
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Prerequisites and ownership checks
- Administrative access to the Resend account that will own the domain.
- Access to the authoritative DNS zone for the selected domain or subdomain. The company that hosts your website is not necessarily the company that hosts DNS.
- A complete list of legitimate senders and the domains they use in envelope-from, DKIM and visible From fields.
- A mailbox or reporting address that can receive DMARC aggregate reports.
- An application configured to use Resend for delivery, with credentials stored server-side rather than in browser code.
DNS changes may be cached according to their TTL. Do not assume that a record is wrong simply because Resend cannot see it immediately; first confirm the record was added to the authoritative zone and that its owner name and value were copied exactly.
Step-by-step: authenticate a domain in Resend
- Select the identity. Decide whether Resend will send from the apex domain or a dedicated subdomain. Use the exact domain that will appear in your application’s From address and keep a written inventory of other senders.
- Add the domain in Resend. Open the domain-management area in the Resend dashboard and add the chosen domain. Resend generates domain-specific DNS records; the names, selectors and values are tied to your account.
- Copy the displayed SPF and DKIM records. Open the DNS provider that is authoritative for the domain. Create the record types, owner names and values exactly as Resend displays them. Do not substitute values copied from a tutorial, another Resend account or a different domain.
- Inspect SPF before saving. If an SPF TXT record already exists, edit that single policy to authorize every legitimate sender. Never create a competing SPF record at the same owner name.
- Publish DMARC. Add a TXT record at the
_dmarcowner name for the domain that appears in From. Start with a monitoring policy while you identify all legitimate sources and review reports. - Wait for DNS visibility. Resend can verify only what public DNS returns. Allow for TTL and provider propagation, then use the dashboard’s verification action. If it fails, compare the authoritative response with the copied record rather than repeatedly creating duplicates.
- Send a representative message. Deliver mail to test accounts at the providers your customers use. Inspect the complete headers for SPF, DKIM and DMARC results, the signing domain and the From-domain alignment.
- Document the final state. Store the domain, selector, return-path choice, report destination and all authorized senders in your runbook. Revisit the inventory whenever a service is added or removed.
How to build the SPF policy safely
Resend supplies the SPF authorization needed for its sending infrastructure. Add that authorization to the one SPF policy for the relevant domain, alongside any other services that legitimately send from the same identity. Preserve mechanisms required by existing senders and remove entries for systems that no longer send.
Common SPF failure modes
- Two TXT records beginning with
v=spf1: receivers may treat the policy as invalid. Merge the mechanisms into one record. - Editing the wrong DNS zone: a record at a secondary provider has no effect if the domain’s nameservers point elsewhere.
- Assuming parent-domain inheritance: a policy on
example.comis not automatically the policy for every subdomain. - Exceeding SPF lookup limits: nested includes and many DNS mechanisms can push a policy beyond the protocol’s lookup limit. Remove obsolete senders and simplify the policy with your DNS or email administrator rather than adding more includes blindly.
SPF authenticates the envelope or return-path domain, not necessarily the human-visible From domain. That distinction is why DMARC alignment must be checked after SPF is passing.
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How DKIM verification works with Resend
Resend creates the DKIM selector and public-key record for each domain. Publish the exact record in authoritative DNS. The private key remains with the sending service; you publish only the public key. A successful Resend verification means the expected key is discoverable, but delivered-message headers still show whether messages are actually being signed with the selector you configured.
When testing, check that the DKIM result is pass, note the signing domain (d=) and confirm that it aligns with the From domain under your DMARC policy. A DKIM pass from an unrelated provider domain can authenticate the message while still failing DMARC alignment.
DMARC policy: start with visibility, then enforce
Create DMARC at the owner name _dmarc for the organizational domain you want to protect. A monitoring record commonly uses the syntax v=DMARC1; p=none; rua=mailto:[email protected]. This is an illustrative format only: choose a reporting mailbox and use the policy syntax appropriate to your domain. Do not paste an example address or policy without adapting it.
| Policy | Receiver request | Best use | Risk if adopted too early |
|---|---|---|---|
p=none |
Deliver normally while sending reports | Inventorying senders, fixing alignment and establishing a baseline | Unauthenticated mail is not actively quarantined or rejected |
p=quarantine |
Treat failing mail as suspicious, commonly by placing it in spam | Staged enforcement after legitimate sources are understood | Valid mail from an overlooked sender may be filtered |
p=reject |
Reject messages that fail DMARC | Strong protection once every legitimate stream is authenticated and aligned | Misconfigured applications can lose mail at SMTP time |
Move from monitoring to enforcement only after reports and representative headers show that every legitimate sender passes SPF or DKIM with alignment. If you change vendors, add a product, or introduce a new subdomain, revisit the inventory before tightening policy.
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Domain authentication and application sending are related but separate tasks. Resend documents a configurable return-path subdomain used for SPF authentication, DMARC alignment and bounce handling. It can be configured through the dashboard or API. The cited Resend changelog described send as the default return-path subdomain at that time; confirm the current default in your account instead of relying on that historical value.
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After DNS verification, configure your server-side application to send through Resend using the verified domain. Keep API credentials out of client-side JavaScript, queue transient failures, and log message IDs, recipient, From identity and provider responses. The material available here establishes the domain-authentication flow, not current SDK method names or request parameters, so consult the current official Resend API documentation for implementation code.
Independent reader supportYour contribution helps us test, update, and keep practical guides available for everyone.Gmail requirements for SaaS senders
Google’s requirements apply specifically to mail sent to personal Gmail accounts. All senders must configure SPF or DKIM. A sender exceeding 5,000 messages per day to Gmail accounts must configure both SPF and DKIM plus DMARC; Google permits a DMARC policy of p=none for that requirement. For direct mail from these bulk senders, the domain in the From header must align with SPF or DKIM.
Bulk marketing and subscribed messages also need one-click unsubscribe and a clearly visible unsubscribe link. Google lists additional expectations, including TLS, valid forward and reverse DNS, correct message formatting and a spam-rate target. Gmail Postmaster Tools identifies a 0.3% spam-rate ceiling in its sender guidance; treat that as an operational threshold to monitor, not as an assurance that mail under the threshold will reach the inbox.
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Authentication is not inbox placement
SPF, DKIM and DMARC prove or describe identity; they do not make recipients want the message. Inbox placement also depends on consent, complaint rates, content, list hygiene, sending patterns, bounce handling, domain reputation and receiver-specific filtering.
Resend requires opt-in recipients and imposes additional conditions for bulk mail. Send only to people who requested the messages, suppress hard bounces and repeated complaints, and make subscription mail easy to leave. A perfectly authenticated message sent to an acquired or stale list can still be filtered or damage the domain’s reputation.
Inspecting a delivered message
Use a representative message rather than relying only on a green verification badge in the Resend dashboard. In the raw headers, locate the receiver’s authentication-results entry and record:
- Whether SPF passed and which envelope domain was evaluated.
- Whether DKIM passed, including the selector and signing domain.
- Whether DMARC passed and which domain aligned with From.
- The visible From domain, return-path domain and any intermediate relays.
Test each meaningful stream separately. A password-reset message may use a different From identity or return path from a newsletter, and passing results for one stream do not prove that another stream is aligned.
Troubleshooting decision tree
Resend says the record is missing
- Check the domain’s authoritative nameservers and confirm you edited that provider.
- Compare the owner name exactly; some DNS interfaces append the domain automatically, while others require the fully qualified name.
- Compare the value character for character, including selector text and punctuation.
- Remove duplicate or stale records that create ambiguity, then wait for the record’s TTL before retrying verification.
SPF fails but the record is visible
- Confirm the message used the domain whose SPF policy you edited.
- Check that every legitimate sender is in the single policy and that no second SPF record exists.
- Review nested includes and lookup-limit errors.
- Check the return-path domain; SPF evaluates that identity, which may differ from From.
DKIM passes but DMARC fails
Compare the DKIM signing domain with the visible From domain. Authentication from an unrelated provider domain is not aligned. If the domains should match, review the From identity selected by the application and the return-path or custom-domain settings in Resend.
DMARC fails after a new vendor is added
Add the vendor to the sender inventory, publish the vendor’s required authentication records in the correct zone, and verify alignment with From. Keep DMARC at monitoring or an appropriately staged policy until the new stream appears correctly in reports and headers.
Messages authenticate but land in spam
Check consent, complaint and bounce trends, sending volume changes, unsubscribe behavior, content and list hygiene. Authentication has already answered the identity question; spam placement requires correcting the recipient and sending signals that receivers evaluate separately.
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Quick Recap
Operational checklist
- One SPF policy exists for each sending identity, with all legitimate sources consolidated.
- Resend’s current account-specific DKIM records are published and verified.
- DMARC is present at the correct
_dmarcowner name and sends reports to a monitored address. - From, DKIM and return-path domains are checked for alignment in real headers.
- Every application and third-party sender is documented before DMARC enforcement is tightened.
- Bulk Gmail traffic is measured against the 5,000-message threshold and the 0.3% spam-rate guidance.
- Marketing and subscribed mail has one-click unsubscribe, a visible unsubscribe link and documented opt-in consent.
- DNS changes, selectors, report destinations and ownership contacts are recorded for incident response.
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