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An email API workflow has three distinct parts: publish the provider’s DNS records to authenticate a sending domain, submit messages through the API, then use webhooks and searchable logs to follow what happened. The API’s acceptance of a request is not proof of inbox delivery, and a delivery event usually means the recipient’s mail server accepted the message—not that it reached the inbox.
What “verified sending domain” means
A sending domain is verified when the email provider recognizes the DNS records it requires for that domain. The domain owner publishes those records at the DNS host; the email API is a separate service that submits messages. Exact record names and values vary by provider and account, so use the values issued in the provider’s current setup flow rather than copying a record from another service or an old tutorial.
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SPF, DKIM, and DMARC have different jobs
- SPF identifies sending hosts authorized for the relevant envelope domain.
- DKIM lets receiving systems validate a cryptographic signature using a public key published in DNS.
- DMARC specifies a policy for messages that appear to use the domain but fail authentication checks.
These mechanisms work together, but their records and alignment behavior depend on the provider and domain configuration. Twilio SendGrid’s domain authentication guide describes setup and the DNS records involved; its SPF, DKIM, and DMARC guide explains their roles.
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Other DNS records can serve different purposes. SendGrid describes link branding as a separate setup using CNAME records so tracked links appear to use the customer’s domain while activity routes back to SendGrid. Its onboarding documentation says reverse DNS is required only for accounts using dedicated IP addresses.
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A dashboard’s “verified” status may not mean every relevant DNS record is currently present. Postmark’s Domains API documents that its DKIMVerified field means DKIM has been verified at some point and can remain true after the DNS record is removed. Check the provider’s current setup guidance and the DNS record state; do not treat a historical boolean alone as proof of present health. Postmark also exposes current DKIM host and value fields and pending values used for rotation. Its SPF verification fields and endpoint are marked deprecated.
How an API request becomes an email event
1. Submit the message
Your application authenticates to the email service and sends a request containing message details such as sender, recipient, subject, and content. As one provider-specific example, Postmark’s Email API requires a registered, confirmed sender signature for the From address. Its endpoint accepts text or HTML content and supports tags, metadata, tracking options, attachments, and a message stream.
A successful API response means the provider accepted the submission; it does not establish that the destination server accepted the message. Postmark’s response includes a MessageID and SubmittedAt timestamp. Save the message ID alongside your own request or user context so you can connect later events to the originating action. Where supported, use tags or metadata for that correlation; field names differ across providers.
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2. Receive lifecycle events through a webhook
A webhook is an HTTP POST from the provider to an endpoint you operate when a selected event occurs. It lets your application react to events without continually polling for updates. Webhook behavior is provider-specific: Postmark’s webhook documentation says it tests each enabled outbound event type and checks for a 200 response before considering it verified. If an event type persistently fails later, Postmark can mark it unverified and pause delivery of that type until the endpoint is repaired and verified again.
Build the handler as a production API endpoint. Validate the payload, correlate it with the message ID, make side effects idempotent so retries do not duplicate work, persist the information needed for diagnosis, and return success after processing or safely queuing the event. Protect the publicly accessible endpoint using the security controls appropriate to your provider. Postmark says its outbound webhooks are not signed and recommends measures including HTTP Basic Authentication and IP allowlisting; those recommendations should not be assumed to describe every provider’s webhook security contract.
3. Interpret the event before acting on it
“Delivered” does not necessarily mean “in the inbox.” Postmark’s delivery webhook documentation puts it plainly: “Delivery means the receiving server accepted your message, but it doesn’t guarantee it reached the recipient’s inbox.”
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Keep the stages distinct: API submission, provider processing, receiving-server acceptance, and events such as a bounce or complaint are different signals. Delivery events are recipient-specific, so a message sent to multiple recipients can produce separate delivery events. Opens and clicks are observations through tracking mechanisms, not definitive evidence of human attention; Postmark’s send API exposes controls for open and link tracking.
How to find a missing message in email API logs
Use webhooks for prompt application reactions and searchable logs for investigation and reconciliation. Start with the message ID from the API response, then inspect the provider’s message details and event history. If you do not have the ID, search with fields your provider supports, such as recipient, sender, tag, status, date range, subject, stream, or metadata.
Postmark’s Messages API supports outbound-message searches using filters that include recipient, sender, tag, status, dates, subject, stream, and metadata, with detail lookup by message ID. Its documentation states that message retention is 45 days by default and can be configured from 7 to 365 days.
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Twilio SendGrid documents an Email Logs API for structured, event-level message history and lookup by message ID. Its documentation states that all email customers have access to 30 days of event history and that this retention cannot be extended. SendGrid also documents an Email Activity Feed with event sequences, search and filtering, and CSV download. Its history add-on provides up to 30 days, and its documentation says activity data is stored in the United States. Confirm feature access, add-on requirements, retention, and regional handling for the account you use.
If you need event history beyond the provider’s retention period, persist relevant webhook events or export them to your own logging or analytics store. Keep only the message and recipient data needed for diagnosis, and apply your organization’s access and retention rules.
What to compare when choosing an email API
Provider documentation describes different contracts and limits, so compare the operational details that affect your implementation rather than assuming a verified domain, webhook, or log works the same everywhere.
Quick Recap
| Area | Questions to check |
|---|---|
| Domain setup | Which DNS records are required? Are custom Return-Path or link-branding records available? Are there dedicated-IP requirements? |
| Webhooks | Which event types are available? How are endpoints verified? What happens on retries or persistent failures? What security controls are supported? |
| Search and correlation | Can you search by message ID, recipient, tag, status, or metadata? Is there API access as well as a user interface or export? |
| Retention and access | How long is history retained? Does longer history require an add-on or plan? Where is activity data handled? |
| Tracking | Which events require open or link tracking to be enabled, and what do those observations actually establish? |
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