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For reliable transactional email, authenticate the domain customers see in the From address, make sure SPF or DKIM aligns with that domain under DMARC, and capture provider events in your application. These steps improve authentication and give your team evidence about what happened to each message, but they do not guarantee inbox placement. A delivery event means the receiving mail server accepted the message—not that it reached the inbox or was read.
What you need to configure and monitor
Transactional email includes messages triggered by application activity, such as account verification, password resets, receipts, and security alerts. A typical setup has three parts: an email-sending integration, DNS authentication for the sending identity, and event handling that records delivery, bounce, and complaint outcomes.
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Amazon Simple Email Service (SES) supports sending through either its API or SMTP interface. Its documentation recommends authenticating email and describes SPF, DKIM, and DMARC configuration. The protocols are shared concepts; the precise DNS records and setup flow vary by provider.
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- Alignment: DMARC checks whether a passing SPF or DKIM result aligns with the domain in the visible From address.
- Operational evidence: provider events tell your application whether a receiving server accepted a message or whether a bounce or complaint was reported.
Amazon SES also documents a reputation dashboard and publication of metrics to CloudWatch. Those tools can help with operational visibility; feature documentation is not evidence that one provider achieves better inbox placement than another.
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How the email domains relate
Three domain identities can appear in an email flow. They have different roles, so a valid record or signature on one does not automatically authenticate or align the others.
| Identity | What it does | Why it matters |
|---|---|---|
| Visible From domain | The domain shown to the recipient as the sender. | DMARC evaluates alignment against this domain. |
| MAIL FROM domain | The envelope-sender identity used in the mail transfer process; it is associated with SPF authentication. | A passing SPF result contributes to DMARC only when its domain aligns with the visible From domain. |
| DKIM signing domain | The domain identified by the message’s DKIM signature, which recipient systems can check against public DNS information. | A passing DKIM result contributes to DMARC only when its signing domain aligns with the visible From domain. |
How to set up DKIM for transactional email
DKIM adds a cryptographic signature to outgoing messages. A receiving system can use the public key published in DNS to check the signature and whether the signed message has been altered. The signing domain is not necessarily identical to the visible From domain, so check alignment as well as whether the signature passes.
Choose a DKIM method supported by your provider
Amazon SES documents four DKIM approaches:
- Easy DKIM: SES manages the keys and signing.
- Deterministic Easy DKIM (DEED): a regional identity can inherit DKIM configuration from an Easy DKIM parent where the setup applies.
- Bring Your Own DKIM (BYODKIM): you supply the key material.
- Manual signing: your sending system creates the DKIM signature.
For a new SES identity, current documentation says Easy DKIM defaults to a 2048-bit key and also supports 1024-bit keys where needed. That is an SES configuration detail, not a universal requirement or a deliverability benchmark. If you send through multiple AWS regions, complete DKIM setup in each sending region unless an applicable regional inheritance arrangement covers it.
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Use the exact records and verification steps shown by your sending provider for the domain and method you selected. Record names and values are provider-specific, and the available information here does not establish universal values to copy. After publishing records, confirm the provider recognizes the identity and that messages are being signed as intended. If you use more than one sending region, check each region separately.
How to align SPF or DKIM with DMARC
DMARC evaluates mail using the domain in the visible From address. It requires at least one passing authentication mechanism—SPF or DKIM—to align with that domain. An SPF record can exist and a DKIM signature can validate, yet neither result may satisfy DMARC if the relevant domain does not align.
Start with monitoring, then assess enforcement
Amazon SES documents a gradual DMARC rollout that begins with a monitoring policy, p=none. Use monitoring to review reports and assess which legitimate mail sources pass or fail before changing the policy. Only advance enforcement after considering the effect on mail flow. A quarantine or reject policy can change how receiving systems handle mail that fails DMARC; setting one prematurely can disrupt legitimate messages.
When an SES custom MAIL FROM domain is useful
SES offers an optional custom MAIL FROM configuration. It requires SPF TXT and MX records on a qualifying subdomain of a verified identity’s parent domain. SES advises that this subdomain not also be used to send or receive email. A custom MAIL FROM domain can make SPF alignment possible, but it does not replace DKIM alignment; either passing, aligned mechanism can satisfy DMARC.
SES also lets you choose what happens if the custom MAIL FROM MX record fails: fall back to the default MAIL FROM domain, or reject sending when the custom domain is not verified. This choice affects SES sending behavior and should not be generalized to other providers, which may have different DNS requirements and failure handling.
What a delivery webhook proves—and what it does not
Postmark’s delivery webhook is triggered after the destination mail server responds OK. In Postmark’s terms, delivery means the receiving server accepted the message. It does not establish that the message landed in the inbox, avoided a later filtering action, was displayed to the recipient, or was read.
Keep that distinction visible in dashboards and user-facing status. For example, label the state “accepted by receiving server” rather than “in inbox” or “read.” Treating server acceptance as proof of inbox placement overstates what the event tells you.
How to handle bounce and complaint events
Configure provider events to reach an application endpoint or event-processing service. Postmark associates webhooks with a message stream and documents triggers for delivery, bounce, and spam complaint events. It tests enabled event types and requires successful verification before delivery; a persistently failing event type can be paused until the endpoint is repaired and reverified.
Preserve the provider outcome
Postmark’s bounce webhook carries processed outcomes that include hard bounce, soft bounce, and undeliverable. Do not collapse every failure into a single ambiguous “email failed” state. Preserve the original provider event and its category, then map it into an internal status your application can explain.
Best Value
- For an invalid or persistently bouncing address, prevent further sends when the provider’s event semantics justify suppression, and give the user a way to correct the address where appropriate.
- For temporary or otherwise recoverable outcomes, apply your policy using the provider’s category rather than treating every bounce as permanent.
- For spam complaints, update sending eligibility and alert the responsible team according to your application policy.
- Use recipient-level outcomes for support, product status, and operational reporting instead of assuming a message-level event describes every recipient.
Build a consumer that can withstand retries
As an implementation practice, store the provider’s message identifier, recipient, event type, timestamp, and relevant outcome details. Make event processing safe to repeat so retries do not create duplicate state changes or alerts. Keep an event trail that lets support and engineering trace what happened, and separate the provider’s raw outcome from the internal status shown to users. These are resilience recommendations, not a claim about a particular provider’s exact retry or delivery guarantee.
Independent reader supportYour contribution helps us test, update, and keep practical guides available for everyone.What to evaluate when choosing an email API
Compare documented capabilities and operational fit, not an assumed deliverability ranking. The available provider documentation supports the following selection questions, but it does not establish head-to-head inbox-placement performance.
Quick Recap
| Area | Questions to ask | Documented examples |
|---|---|---|
| Sending integration | Can your application send through an API, SMTP, or both? | SES documents both API and SMTP sending. |
| Authentication controls | Which DKIM methods are supported? What SPF or custom MAIL FROM setup is available? How do you configure DMARC alignment, and does setup vary by region? | SES documents Easy DKIM, DEED, BYODKIM, manual signing, custom MAIL FROM, and region-specific considerations. |
| Event evidence | Are delivery, bounce, and complaint events available? What does each event actually mean? How are endpoints verified, and what bounce details are exposed? | Postmark documents delivery, bounce, and spam complaint webhook triggers, along with event verification and bounce outcomes. |
| Operational visibility | Can your team inspect reputation and sending metrics or connect them to existing monitoring? | SES documents a reputation dashboard and CloudWatch metric publication. |
Validate the setup before relying on it
- Choose the integration and sending identity. Select your provider’s API or SMTP interface, verify the sending identity, and identify every region from which your application will send.
- Configure DKIM. Select a supported method, publish the provider’s DNS records, and verify the identity in each applicable region.
- Check DMARC alignment. Confirm that at least one passing SPF or DKIM result aligns with the visible From domain. If using SES custom MAIL FROM, publish its required SPF TXT and MX records on the qualifying subdomain and select the MX-failure behavior.
- Roll out DMARC carefully. Begin with monitoring at
p=none, examine reports and legitimate mail flow, then consider stronger enforcement only after assessing the consequences. - Connect event handling. Configure delivery, bounce, and complaint events supported by your provider. Complete endpoint verification and monitor event delivery so a failing trigger does not go unnoticed.
- Test application behavior. Confirm that your system records recipient-level outcomes, handles repeated events safely, distinguishes server acceptance from inbox placement, and applies suppression or correction workflows based on the provider’s event meaning.
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